Immigration New Zealand says it is extremely concerned by the suggestion Tongan people have been accepted into the country after having their criminal convictions wiped in Tonga.
The Pacific nation's police chief confirmed this week that at least two officers have been involved in signing letters clearing people of their criminal records.
Immigration New Zealand says it has stepped up its checks on police certificates from Tonga.
Pacific regional manager Alan Barry says the Government is working with the Tongan authorities to get the details of people who may have come to New Zealand after having their record cleared.
Mr Barry says anyone who is found to have entered New Zealand illegally by not declaring criminal convictions or by providing misleading information to an immigration officer will be liable for deportation and could face a lengthy ban on ever returning.
Tonga's police commissioner Grant O'Fee says he expects the number of letters signed by officers stating that people with convictions have a clean record will exceed 50.
Mr O'Fee says one of his main concerns is establishing how widespread the practice was between 2005 and 2010.
He says he has other concerns, including claims by officers that a previous police commissioner approved the practice. That matter is now in the hands of the Solicitor-General.
Mr O'Fee says he has put a stop to the practice.