Prime Minister Jacinda Ardern says she is deeply disappointed in Transport Minister Phil Twyford for making a phone call on a plane after the aircraft doors had shut.
Mr Twyford made a one minute phone call to one of his staff while on a plane preparing for take off from Wellington airport on 17 May, for which he said he apologies unreservedly.
He informed the Prime Minister of this yesterday afternoon.
"My understanding was that he went back, looked at his phone records, realised what he had done and then contacted me," Ms Ardern told Morning Report.
The story came to light after National transport spokesperson Judith Collins lodged a written question about the incident.
Mr Twyford offered his resignation to Ms Ardern, who declined his offer but instead stripped him of his Civil Aviation Authority (CAA) portfolio.
"We are required, and indeed have to maintain, a high standard and make sure that we follow the rules."
She said she cannot determine whether it was obvious to the minister that he had in fact broken the rules until the issue was raised with him.
"He's broken the rules, he has fully admitted that what he has done was wrong, he has undertaken to make all of the information available to CAA, there is no excuse."
Ms Collins said the transport minister did the right thing in offering to resign.
Ms Collins was alerted to the incident by someone from the flight, and said Mr Twyford's actions were a very serious breach of the rules.
She said the public would expect the transport minister to obey the law on an aeroplane.
The incident has been referred to the CAA.
The Civil Aviation Authority said mobile phones do pose a safety hazard to aircraft as they can interfere with its electrical systems.
It said there have been a number of incidents where electronic devices have interfered with an aircraft's instruments.
But the aviation writer Peter Clark said the current rules hark back to a time when people didn't know much about the energy emitted from cellphones.
"I think it is widely known now that it possibly does not happen, or [only] in extremely remote situations, but the requirement is there to turn your phone off its transmitting mode, in the cellphone mode, when the door closes," he said.
"So that is the requirement, that is the law and I suppose you need to really adhere to that law."