The $2.5m will be on top of the $5m New Zealand has already committed to help Tongan police reform after public confidence in the force was dented by the 2006 Nuku'alofa riots.
Eight people were killed and about 80 percent of the capital's business heart was wiped out in the rampage.
Police and tourism were key areas of focus for New Zealand's assistance, said Prime Minister John Key, who is this week on goodwill visits to Tonga, Samoa, Cook Islands and Niue.
New Zealand was committed to Tonga's political reform process and economic progress, he said after talks with Tonga's Prime Minister Dr Feleti Sevele yesterday.
Tonga has a semi-feudal political system in which the king has the power to appoint a prime minister and cabinet. King George Tupou V supports the political reform, which could see an elected parliament make such appointments.
Mr Key praised the prime minister and king for their part in the process.
To support reform New Zealand's annual aid programme to Tonga will increase from $12m to $16 million in 2009/10 and 2010/11, and to $18 million in 2011/12, to fund a civic education programme.
Mr Key said he and King George discussed the changes to the constitution and New Zealand's opportunity to contribute in Tonga, the New Zealand Herald reported.
He said the king was "very optimistic" that things were on track.
The final version of a report into constitutional reform is expected in November and elections would be held in 2010.
Mr Key said King George was confident there would be no repeat of the 2006 riots.
The trip is also gauging how Pacific countries are coping with the economic recession and will set priorities for talks at the Pacific Island Forum in Australia in August.
Dr Sevele briefed Mr Key on progress made as a result of New Zealand aid, Radio New Zealand reported.
Most Tongans had land on which to live and plant food, the effects of the recession are not as bad as they could have been, Dr Sevele said.
