A police jeep interrupted our friendly Sunday afternoon game of soccer by driving onto the rocky and uneven field. Behind the wheel was the only police officer in Pangai, the capital of Tonga's Ha'apai group. The lawman drove right up to where I stood with my foot on the ball and slowly began shaking his head. "No...no...no!" he said, "You can't do that here on a Sunday". And he was right. Tongan society is extremely religious and their law prohibits people to work, play, swim, fish or trade on Sundays. So, after a little grovelling and some vigorous apologies, the Sergeant let us leave with only a stern warning.

With our illegal soccer game ended the troublemakers and I slunk back to the various yachts we were staying on at the very basic Pangai marina. Unfortunately, when the police officer had to come out that night to stop us breaking the Sabbath laws again he wasn't so friendly. On reflection we probably should have expected that a sing along accompanied by guitars, drums and a didgeridoo would attract attention in such a quiet backwater port in the South Pacific.

I was in this isolated part of Tonga aboard the small sailing vessel, Destiny, with three others. We had sailed there to absorb the peaceful way of life in one of the quietest corners of the Pacific, where modern luxuries are few and far between. Indulgences such as electricity are uncommon. Consequently there is very little tourist activity in the Ha'apai group and only one or two resorts. To properly explore the 68 mostly low-lying coral atoll islands of the Ha'apai group you need a boat. Preferably a yacht; to make use of the abundant trade winds that caress the Tongan island chain. Sure, you could take a 45 minute flight from the Tongan capital of Nuku'alofa to the Ha'apai group. But without a yacht to cruise the mile upon mile of coral-fringed, white-sanded, idyllic islands you might as well not bother. Sailing is the only way to truly experience this part of the Kingdom of Tonga.

Our crew weren't such delinquent lawbreakers every Sunday. On the contrary, once we even attended a Wesleyan church service at the invitation of a local parishioner. You see, no visit to Tonga is complete without at least one Sunday church service in order to hear their spiritually moving singing. Our choral experience took place on the island of Ha'afeva, 30 miles southwest of the capital, Lifuka. As huge brass bells rang out across the island calling worshippers to church, we quickly took a seat in one of the back rows. Of course, the service was in Tongan, so apart from the unmistakable meaning in some very impassioned sermons, the female preacher's words went right over our heads. But we hadn't come for the sermons, we had come for the singing. The sheer volume of their voices was stunning enough, but when combined with the multi-part harmonies their singing had a powerful and spine-tingling presence. We were fortunate enough to be seated in front of several large Tongan gentlemen whose baritone voices had the most sublime depth. There was a certain level of resonance they achieved that made the hairs stand up on our necks and left us in no doubt of the strength of their faith. The church hall reverberated with a pure vocal energy we had never heard the likes of before.

After church we were invited for Sunday lunch at the home of a local woman. Holding her tiny children's hands we wandered down the dusty dirt track through the centre of Ha'afeva village towards their house. The family home was a humble corrugated iron shack not much larger than a medium-sized bedroom, blackened from an open fire at its far end. As our host's children swarmed playfully over our knees we were served dinner at the rickety kitchen table. Our meal had been cooked in an underground oven the Tongans call an Umu. We were served a Sunday favourite – roasted tapioca root, sun dried whole fish and a South Pacific specialty: tinned corned beef, drowned in coconut milk and wrapped in taro leaf packages. All this was washed down with a whole coconut full of sweet milk. When we decided to depart our hosts continued their generosity, giving us fresh papayas, bananas, taro and purple yams to take back to our yacht. It is this sort of incredible hospitality and graciousness that makes you realise why Tonga was long known as the Friendly Islands.

Later that day we met a volcanologist and his wife, who had also sailed from New Zealand. We mentioned that we wanted to visit the active volcano island of Tofua, which stands out like a beacon among the surrounding flat islands. He told us he had spent many years studying volcanoes in New Zealand and Indonesia, where scientists carried masks and gas measuring devices to protect themselves from the toxic gases produced by volcanic activity. "Hydrogen Sulphide gas can kill you in three minutes," he told us solemnly. "It is heavier than air, so it sinks and sits in hollows. It smells like rotten eggs at first, but as the gas takes effect it dulls your senses, so you can't smell it any more. Then you collapse and die." This sobering information didn't dampen our enthusiasm to visit Tonga's most active volcano, which last erupted in 1958. However, we were sure to stay upwind and to run like hell if we smelled rotten eggs.

Except for occasional visits by farmers from nearby islands coming to tend their kava crops, Tofua is deserted. Kava, a popular natural sedative in Tonga and Fiji, thrives in the rich volcanic soil of the island. Tofua's historical claim to fame is that it was the site of the mutiny on the Bounty. In April 1789 Captain Bligh's command was taken at knifepoint by Fletcher Christian's fed up mutineers. Bligh and his officers were set adrift with few rations and reached Tofua's steep and rocky shores to search for water. Here they were attacked by the locals and barely escaped. The Bounty's quartermaster, John Norton, was clubbed to death at a spot now imaginatively known as Mutiny on the Bounty Beach. Present day visitors come to Tofua to climb through the steep rainforest, thick with lush ferns, to reach the rim of the active volcano. Upon arriving at the summit you are rewarded with an impressive view down into the massive crater lake, which takes up most of the interior of the island. For the ultimate visit to Tofua you can catch a seaplane from Nuku'alofa and actually land on the crater lake.

Travellers are such an unusual sight in the more isolated Ha'apai islands that invariably upon coming ashore you are met by a horde of local children. They delight in leading you around their village by the hand in a huge convoy of smiles and always ask the same two questions in English; "What is your name?" and "Where are you from?". At Nomuka island – one of the most isolated and picturesque islands in the Ha'apais – we were followed by a procession of local kids, two of whom rode beautiful white horses. They giggled and smiled as they led us proudly around the dirt streets and up to the highest point of their island.

A large, landlocked lagoon dominates Nomuka's centre. From our vantage point on the hilltop overlooking the village we saw majestic Ikales (Tongan harriers) circling and floating on thermal air pockets above the shallow lagoon. Later we discovered a colourful local church surrounded by the fragrant blossoms of Frangipani trees. Because there are no regular ferries and only occasional visits by supply boats, Nomuka offers the true experience of Tongan culture as it has remained – mostly unmolested – for centuries. And this is precisely why sailing around the Ha'apais by yacht is the only real way to explore this part of Tonga.

More whales visit the Kingdom of Tonga than most other South Pacific nations. And we found the Ha'apai group had more of the magnificent mammals than anywhere else in Tonga. The leisurely pace of sailing around the Ha'apais is the perfect way to really get your mind and body in sync with the slow tempo of Tongan life. So, a common six hour sail to travel 30 miles between islands is a great opportunity to spot whales leaping on the horizon. Usually seen two or three at a time, the colossal mammals slowly cruise through this part of the Pacific in the winter months. Whale watching in Tonga is centred mainly on the giant Humpback Whale, which migrates to Tonga to breed and give birth. We saw small groups of Humpbacks and other whales almost every time we set sail. While cruising we were occasionally treated to a close-up view of the relaxed sunbathing giants. They would keep pace with Destiny, attracted to her size and noiseless movement through the water. On two occasions the whales swam towards us for a closer view. It was a humbling feeling to see their dark, wise eyes inspecting our faces before gently slipping under the water and disappearing. Whale watching is a two-way encounter. It seems humans and whales are equally curious about each other.

Ha'apai's capital city of Pangai, on the island of Lifuka, used to be the capital of all Tonga. It was the seat of the King, from where he controlled an empire covering a vast area of the South Pacific. Today Pangai has a population of only 1700 and is as sleepy a South Pacific town as you can imagine. It boasts one poorly-stocked supermarket, a telephone exchange, a few small stores and what is unquestionably the centre of town; Mariner's Café. This restaurant/bar, run almost single-handed by an Australian named Trevor, is the only place to get a cooked meal and enjoy a cold drink in Pangai. It is budget fare, but after having to cook for ourselves for weeks the burgers and stir fries were delicious. One great advantage of Mariner's Café being the only eating establishment in town is that it has also become the only gathering place. Everyone, from backpackers to yachties and locals must congregate here – and they do. Mariner's Café is the social centre of Pangai. Trevor, the owner of the café and its connected guesthouse, is something of a local legend. He is the man to talk to if you need to know anything about Lifuka and its attractions.

On Trevor's advice we travelled north from Pangai, crossed the airstrip (watch out for incoming flights!) and then negotiated the narrow, potholed causeway to reach the connected island of Foa. Here we found the most stunning beach in the Ha'apai group, if not all of Tonga. The blandly named Sandy Beach sits at the northern end of the island, featuring coarse white-gold sand that contains a myriad of tropical shells. Wandering the shoreline here is a beachcomber's dream. Sandy Beach becomes a narrow peninsula surrounded by a reef enclosing warm azure waters. Here there is excellent snorkelling territory to explore in the crystal clear inner lagoon. You can expect to see hundreds of brightly coloured fish and maybe even a shy octopus or aggressive Moray eel hiding in the crevices of the volcanic reef. You can also watch local fishermen in dug out canoes casting their nets outside the reef, as they have done for centuries. Or, if you're feeling energetic, you could climb one of the coconut palms fringing the water and knock down their fruit. It is then a not-so-simple matter of husking the coconut using a sharpened piece of wood stuck in the ground. Then, using a rock (or ideally a machete) you crack open the coconut and enjoy the milk and crunchy white flesh.

Moments like this will remain with you long after you've departed the Kingdom of Tonga.