The head of Save the Children Australia says the end of its tenure on Nauru means there is no longer an agency to speak up for the rights of children in detention there.
The charity winds up its operations at the Australia-run asylum seeker detention centre today, after the Australian government granted the welfare contract to the multinational, Transfield Services.
The charity's tenure has been controversial, with Canberra last year deporting nine workers after they were accused of encouraging detainees to self harm and fabricate stories of abuse.
An independent inquiry later found no evidence of this.
Chief executive Paul Ronalds said they're leaving the island with mixed feelings.
"We're incredibly proud of what we've achieved, the education we've been able to deliver to asylum seekers and refugees, the support we've been able to provide," he said.
"On the other hand, we are very concerned that with Save the Children leaving, it means there's even one less voice on Nauru to speak out for children and their families."
There needed to be far greater transparency and oversight of Australia's offshore processing centres, which have been condemned by human rights groups and the United Nations, he said.
Mr Ronalds said Save The Children was an easy group to target on Nauru.
"The other organisations are Transfield, which are much larger, multinational, or Nauruans. So we're the easy target, as we've been before," he said.
He said Save The Children's public position of opposing the mandatory detention of children also led to ongoing tension with the Federal Government.
"The Government undoubtedly didn't like us giving evidence to the human rights inquiry, but that was something that we felt was a really critical opportunity for the Australian public to know what was going on with taxpayer-funded facilities on Nauru," he said.
"But it absolutely created further tension with the Government."
Mr Ronald said he was proud of the changes Save The Children made on Nauru, particularly when it came to education within the processing centre.
But he said an independent body needed to be urgently set up to monitor the situation on Nauru.
Immigration Minister Peter Dutton's office was contacted for comment, but he was not available.
Charities asked to pay bonds
Charities working in Australia's asylum seeker detention centres have been asked to pay multi-million dollar bonds that can be forfeited if they speak out against government policy.
Fairfax Media reported that aid agencies, including Save the Children and the Australian Red Cross, were asked by the immigration department to offer "performance security" during contract negotiations.
Mr Reynolds said his organisation refused to sign the bond agreement, interpreting it as a gag clause.
But other organisations, including the Red Cross and Transfield Services, agreed to pay.
The immigration department has denied that the agreements are used to silence non-profit organisations.
Canberra has increasingly sought to maintain secrecy at its controversial detention centres, including passing a law that sanctions staff who disclose information about conditions with up to two years in prison.