Greens MP Jenny Leong raised the matter in state parliament on Wednesday, questioning why NSW was the only Australian jurisdiction to prevent the Subcommittee on Prevention of Torture delegation from assessing jail conditions.
She asked the premier during Question Time on Wednesday: "What exactly are you hiding and what don't you want them to see?"
Mr Perrottet defended his government's decision, saying the NSW prison system "is the strongest in the country".
"Our prison system is there ultimately to do one thing and that is keep the people of NSW safe," he said.
The subcommittee delegation's visit is the first to Australia since it ratified the Optional Protocol to the Convention Against Torture in 2017.
Under its mandate, the subcommittee can carry out unannounced visits to all detention facilities and conduct private interviews with people deprived of their liberty without witnesses.
When pressed again by Ms Leong on reasons for denying access, the premier noted "significant oversight processes" of jails were in place.
"We've set up those oversights in our prison system and if people have issues ... you can raise them with the minister or the corrective services commissioner," Mr Perrottet added.
"If issues are raised in our prison system, they can go to the appropriate channels."
The government's decision was slammed by Indigenous activist Paul Silva, the nephew of Dunghutti man David Dungay Jr of Kempsey, who died in Sydney's Long Bay jail in 2015 after guards rushed his cell.
"I can tell you why the NSW government won't let United Nations inspectors in, they can't deal with scrutiny," he said.
"My uncle David Dungay Junior 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."
AAP has sought comment from the UN delegation.
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