Former Dunedin policeman Chris Kelley was the first foreigner appointed to run the Pacific kingdom's police force in 2008.
He took on the job in the wake of the 2006 Nuku'alofa riots, charged with overhauling the force as part of a five-year development project, jointly funded by New Zealand, Australia and Tonga.
Even though there was still work to be done, the new government decided not to renew his contract for another two years but to advertise for a new person to take over from him. He finishes on Friday.
According to Tonga's Taimi Media, earlier this year about 200 officers, or half the police force, signed a petition to Cabinet expressing dissatisfaction with Mr Kelley's leadership - upset with the investigation and disciplinary process of alleged misconduct of members of police.
The petition organisers also accused Mr Kelley of favouritism toward certain individuals and said the new Police Act was "selfish" and a "dictatorial" piece of legislation.
Mr Kelley, 61, said he was not given any reasons for his contract not being renewed. The petition may have had an impact but that would only be speculation, he told NZPA today.
He said the petition organisers had never come forward and remained "nameless phantoms".
"I believe change was at the heart of that particular initiative, I think change because of disciplinary issues, change because of promotional issues and simply because some people grapple with change, especially if they have been with an organisation for a long time."
Local media reported the Tongan police minister, Viliami Latu, saying the time was right for a local to re-assume command of the force.
However, 3News reported tonight that it understood it was the police minister who wanted Mr Kelley gone. Mr Latu himself was charged with assaulting his wife last year just before Tonga's general elections.
Mr Kelley has also put 16 of his fellow police officers before the courts on charges including theft, violence and bribery
Mr Kelley said he was not bitter but was disappointed the opportunity to continue was not there.
It was not a foregone conclusion he would have stayed for another two years, but it was something he would have seriously considered, he said.
Nevertheless, he said he would leave satisfied the development programme had introduced a great platform for Tonga's police and the new Police Act was "cutting edge", not only for the Pacific, but by world standards.
When he arrived the force was still struggling with the aftermath of the riots and prosecutions and public feelings.
In three years there was an increase in trust in police - there had been an increase in reporting rates for all crime types and reduced road crashes and fatalities, he said.
Changes to liquor rules had improved behaviour and the introduction of breath-testing had been an "unqualified success".
Mr Kelley will return to New Zealand the day after finishing on Friday. He plans to find another job.
"I have a year or two left in me yet."
