The family of a Dunghutti man who died in Sydney's Long Bay jail have called for action to stop deaths in custody after the New South Wales government revealed it would refuse access for United Nations inspectors to state prisons.

David Dungay Jnr died in custody in December 2015 three weeks short of parole after guards dragged him into a cell and held him face down while injecting him with a sedative.

His nephew Paul Silva said by denying independent inspections the NSW Government had something to hide.

"I can tell you why the New South Wales government won't let United Nations inspectors in - they can't deal with scrutiny," he said.

"My uncle David Dungay Jnr died at the hands of six men in a workplace environment and yet Safework NSW and the Department of Public Prosecutions won't even investigate it.

"They want to hide their cruelty and inhumanity from the international community and the wider public."

Mr Silva said police should not be investigating police.

Paul Silva at the June 6 2020 Black Lives Matter protest in Sydney.

NSW Premier Dom Perrottet made comments on the decision to block UN Prevention of Torture Officers from the state's prisons at a press conference on Tuesday.

"We have a custodial inspector in place and we have an ombudsman in place," he said.

"We have raised out concerns (about the proposal of a UN inspection) with the federal government.

"The advice I have received from Corrections here in our state is there are security and operational concerns in relation to the matter."

Last year Mr Dungay Jnr's mother Leetona Dungay said she would make a complaint to the UN regarding the failure of both the NSW and Federal governments to protect her son's life, and for their failure to take action towards ending Aboriginal and Torres Strait Islander deaths in custody.

On Wednesday Mr Silva called on the Federal government to implement all of the recommendations from the Royal Commission into Aboriginal Deaths in Custody.

More than 500 Indigenous people have died in custody since the Royal Commission brought down its findings over 30 years ago.