Phil Twyford offers resignation as Transport Minister

Minister of Transport Phil Twyford has apologised unreservedly to Prime Minster Jacinda Ardern for taking a phone call on a plane.

He made the phone call on a domestic flight after the doors had closed in preparation for take-off.

Civil Aviation Authority (CAA) rules say a person "may not operate, and operator or pilot-in-command may not permit operation of, cellphone or other portable electronic device designed to transmit electromagnetic energy."

People found guilty of this can face a fine ranging between $500 and $2500.

Mr Twyford has offered his resignation as Transport Minister to Ms Ardern, but it was not accepted.

She did, however, re-allocate his responsibility for the CAA to Associate Transport Minister Julie Anne Genter.

The minister's voice broke as he spoke to reporters about the incident, saying: "I made a mistake."

"I clearly wasn't thinking straight at the time and I recognise that. It was unacceptable and I apologise unreservedly."

Mr Twyford says the phone call was to a staffer and took place on a flight on Budget day.

"I clearly at the time thought it was an important matter, but in hindsight it doesn't excuse or justify breaking the rules."

The Prime Minister, clearly not impressed with her Transport Minister, has stripped him of his CAA responsibilities.

"I expect all my ministers to act in accordance with the rules. As Transport Minister it is even more important that Phil abides by civil aviation laws."

"It isn't appropriate for him to have responsibility for the Civil Aviation Authority in the event that it investigates this incident, so it's appropriate to transfer those responsibilities to Julie Anne Genter."

Ms Ardern has not decided whether that is a final decision, or if he could regain the responsibility, a spokesperson told Newshub.

Any outcome of any potential CAA investigation would not have any effect on the decision to reinstate his responsibility, the spokesperson said.

The matter was uncovered by Twyford's political opponent, National's Judith Collins, through a series of parliamentary written questions - including asking the minister whether he was on the phone while the plane was moving or during the safety video.