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 on Saturday, after the Australian government granted the welfare contract to the multinational, Transfield Services.
A recent Senate report outlined allegations of child abuse and sexual assault against Transfield staff on Nauru and found the contractor hadn't been adequately accountable to border authorities.
But Save the Children's tenure has also 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.
Its chief executive Paul Ronalds says 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."
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.
Mr Ronald said he was proud of the changes Save The Children made on Nauru, particularly when it came to education and support services for people at the processing centre.
Charities asked to pay bonds
Meanwhile, Mr Ronalds says Save the Children Australia refused to sign a contract that would have seen it pay a multi-million dollar bond that could be forfeited if it spoke out against government policy.
Fairfax media reports that charities that work with asylum seekers in offshore detention centres were asked by the immigration department to offer "performance security" during contract negotiations.
The department has denied that the agreements are used to silence non-profit organisations.
Paul Ronalds says Save the Children refused to sign the contract and instead negotiated one that allowed it to speak out if it was concerned.
He says the performance bond was not overtly a gag clause, but it was concerning.
"It did contain very strict conditions around how we would deal with the media. And in our view there was a real concern that it would be used to dissuade us from speaking out publicly."
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.