Air New Zealand has admitted a plane carrying international returnees then picked up domestic passengers before being deep cleaned.

In a statement, the national carrier said the incident happened " a number of months ago".

Air NZ operational integrity and safety officer captain David Morgan said the aircraft, operating as an MIQ charter flight, had been "inadvertently scheduled to continue operating instead of returning to Auckland to be cleaned, which is the usual process for these types of flights."

Morgan said the event prompted the airline to review its processes to prevent it happening again.

"This included checking MIQ charter flights are appropriately flagged in our system and that the specific type of cleaning required immediately after flights of this nature has been scheduled.

"We continue to review our processes on an ongoing basis to make sure our customers and our people are kept safe."

He said crew members were asked to report events through the safety reporting system and there had been no further incidents of that nature.

"There have been no cases of customers or aircrew contracting Covid-19 from aircraft surfaces while travelling on subsequent domestic flights following MIQ charters."

The Ministry of Health said it was aware the incident had happened.

In a statement, it said passengers on MIQ charter flights were required to wear a mask for the duration of the flight to prevent droplets being spread through the air or onto aircraft surfaces.