The airport and key assets were immediately secured on request after Tongan authorities failed to quell a riot which killed at least eight people last week.
About 150 Australian and New Zealand soldiers and police have been deployed in Tonga.
Last Thursday, 80 per cent of central Nuku'alofa was torched and looted by a mob, hitting businesses hard.
But the presence of foreign troops on Tongan soil has sparked calls to Canberra and Wellington to withdraw immediately or face accusations of suppressing democracy.
'Akilisi Pohiva, a local legislator and pro-democracy campaigner, says the troops were seen as supporting the government of Fred Sevele, the prime minister of Tonga.
Winston Peters, the foreign minister of New Zealand, said the troops were only in Tonga, about 2,000km north of New Zealand, to ensure the smooth passage of reforms.
"Our presence is not about taking sides. New Zealand has been fully supportive of peaceful democratic reform in Tonga.
"It is clear that the Tongan government is intent on pursuing reform. That was demonstrated last week with its announcement on the direct election of parliamentarians in 2008," Peters said.
In May last year, Tonga saw unprecedented protests when 10,000 protesters, about one-tenth of its population, took to the streets demanding democracy and public ownership of national assets.
