The notion of Tonga being "discovered" by the 17th century navigator Abel Janszoon Tasman may be slightly far-fetched, given that the island cluster has been inhabited by the people of Polynesia for around 3000 years. However, Tasman was the first recorded European to land here, and his reports and charts of the region brought the islands to the attention of the rest of the world.
In fact, the island of Tongatapu (named "Amsterdam" by Tasman due to its abundance of supplies) had been considered by the Royal Society as a possible location for viewing the 1769 transit of Venus.2 Had Wallis not returned to England with news of the newly charted King George's Island (Tahiti) a mere three months before Cook set off in Endeavour, the history of South Pacific exploration may have turned out very differently.
As it was, Cook did not encounter the Tongan islands until his Second Voyage, when he stopped at both 'Eua and Tongatapu (or, by Tasman's nomenclature, Middleburg and Amsterdam respectively) in October of 1773. Here he was "welcomed a shore by acclamations from an immence [sic] crowd of Men and Women not one of which had so much as a stick in their hands".3
Indeed, Cook found the islanders to be so accommodating that he returned to the archipelago in 1774 on his way back from New Zealand. Stopping at the island of Nomuka, Cook was sought out by name, and with this "proof that these people have a communication with Amsterdam ", the cultural unity of the islands was established.
It was at this time that he famously named the island group the Friendly Archipelago, "as a lasting friendship seems to subsist among the Inhabitants and their Courtesy to Strangers intitles [sic] them to that Name." 4
Cook's Third Voyage also included a visit to Tonga, this time for a stay of several months. Cook first dropped anchor at Nomuka in May, and then, at the invitation of the great chief Finau, travelled to another island, Lifuka. Here, Cook and his men were treated to such entertainments as "whould [sic] have met with universal applause on a European Theatre ".

Unbeknownst to Cook, Finau was plotting to murder Cook and his men, and then loot his ships. The plan was to lure the crew to a convenient location and then kill them. However, due to fortuitous timing and in-fighting among the local chiefs the plot was never executed, and Resolution and Discovery escaped unmolested.
It is perhaps one of the great ironies of history that the land which had inspired such warmth in Cook as to bestow on it the moniker of the Friendly Islands was also the site of a plot to kill him!
The truth was only revealed upon the publication of William Mariner's account of his years spent in Tonga;5 Mariner himself was on board Port au Prince when it was treated in just the manner that was once planned for Cook's ships.
Finau also perpetrated another great lie when he dissuaded Cook from accompanying him to Vava'u in the north, telling him there was no suitable anchorage or harbour. In fact, the island of Vava'u is host to one of the world's greatest harbours, the Port of Refuge, and today proves a popular destination for yachties and sailors the world over.
However Cook was never to discover the waters of Vava'u, for he sailed southwards, arriving in Tongatapu in June. The ships remained in Tongatapu for a further month, and during that time Cook described many of his interactions with the Polynesians. Cook's journals, along with the account of William Mariner, provide the best documentation of pre-Christian life in the region, and as such are a valuable record.
Isa Menzies
I work as the volunteer Heritage and Project Interpretation Officer for the Ministry of Tourism in Tonga through the Australian Youth Ambassadors for Development program, an Australian Government AusAID initiative.
References
Originally published in Cook's Log, page 30, volume 32, number 1 (2009).