The revelation in a Senate hearing on Tuesday came as a team of doctors described the severe development and physical problems of children on Christmas Island, including rotting teeth and severe skin infections.

In the Senate hearing, the secretary of Immigration Martin Bowles confirmed the funding for legal costs to thwart opposition to the detention centres came under the memorandum of understanding that has been signed by PNG. Between $350,000 and $370,000 had been spent, the estimates hearing was told.

PNG is fighting a constitutional challenge initiated by the country's Opposition Leader, Belden Namah, against the agreement to process and resettle asylum seekers.

The Nauruan government has also tried to shut down two human rights inquiries that were initiated by human rights lawyer Justice David Cannings.

''The fact our government pays another country's legal bills, while at the same time refusing to fund legal assistance for asylum seekers in Australia, shows how determined this government is to deny asylum seekers a fair go,'' the director of legal advocacy at the Human Rights Law Centre, Daniel Webb, said.

Immigration spokeswoman for the Greens, Sarah Hanson-Young, said the use of public money to stifle human rights investigations was unacceptable.

''The Abbott government is trashing Australia's human rights record while using taxpayer money to pay for Papua New Guinea's constitutional battles,'' she said.