The Transport Minister has apologised for making a phone call on a domestic flight when the doors had closed to prepare for take-off.

Phil Twyford says this is inappropriate for anyone, but particularly for him.

He says he offered his resignation to the Prime Minister, but she declined.

However, his responsibilities for the Civil Aviation Authority have been passed over to the Associate Minister of Transport Julie Anne Genter.

Tywford says he has referred the matter of his phone call to the Civil Aviation Authority, who will follow whatever processes they deem appropriate.

It has been revealed that National's Judith Collins had asked a parliamentary written question on the matter, after which Twyford fessed up.

"It was a breach of Civil Aviation rules, that's unacceptable. I informed the Prime Minister of this matter and offered my resignation."

He says that the call was to one of his staff.

"I clearly at the time thought it was an important matter, but in hindsigh,t it does not excuse or justify breaking the rule."