A top government official has admitted the lapse of legislation designating Nauru as a regional processing hub was a mistake by public servants.
Home Affairs department secretary Mike Pezzullo said the sunsetting of the laws in October last year occurred "as a result of human and administrative error within the department".
He said the department was notified of the sunsetting on several occasions from January 2021, including on October 1, 2022, when the instrument designating Nauru lapsed.
"However, it failed to adequately monitor, track and report on the sunsetting of the instrument that designated Nauru as a regional processing country," he said.
"It shouldn't have occurred."
Mr Pezzullo said the minister "sought and was provided assurances" relevant legislation for Operation Sovereign Borders was in place ahead of the lapse.
The omission occurred during seven ministerial briefings.
Home Affairs Minister Clare O'Neil was subsequently left in the dark about the lapse until mid-December and the error was fixed this month.
Mr Pezzullo said immediate action was taken by the department to ensure it didn't happen again.
"These measures include a new reporting regime and regular spot checks," he said.
Senators and the home affairs chief went head to head over whether the government had the legal authority to detain people on Nauru when it wasn't designated as a regional processing country.
Liberal senator Paul Scarr said it was "difficult to believe there was no legal risk or operational risk posed by the lapsing of this instrument".
"(The law) actually says there is no power to take a person to a regional processing country or an offshore destination of that type unless we've designated a country as a regional processing country," he said.
Mr Pezzullo said he had legal advice stating the government would be covered in the case of any arrivals.
"We're confident that alternative authorities were available to us and that's based on advice," he said.
But he would not reveal the contents of the advice or which other legislation it referred to.
Thirteen people were transferred from Nauru to Australia for medical treatment or accompanying family members during the time of the lapse.
No one was transferred back to Nauru during that time. The department was confident they also had the legal authority to keep them detained in Australia.
The number of people on Nauru has decreased to 66 as of February 1, compared to 111 at the end of August 2022.
"Once this caseload is resolved through current third country resettlement, the regional processing capability will remain ready to receive any new unauthorised maritime arrivals should that occur," Mr Pezzullo added.
