A furious Nauru has accused New Zealand of undermining the Pacific island's sovereignty and trying to influence a criminal investigation.
The New Zealand Government has cut $1.2 million of aid to Nauru's justice system because of human rights abuses.
Nauru opposition MP Roland Kun - who wants to join his family in Wellington - is unable to leave because his passport has been cancelled.
Nauru's Justice Minister David Adeang said Mr Kun was being investigated for trying to overthrow the government.
He said New Zealand's decision to cut aid was misguided and based on mis-information from Mr Kun and his wife.
Mr Kun said he was happy New Zealand had acknowledged the problems with the lack of rule of law and the Nauru government's disregard for human rights.
"There really is no point in assistance towards strengthening the justice system, if what is happening on the ground is the opposite of the that objective."