Transport Minister Phil Twyford will pay a $500 fine for using his phone on a plane after the doors had closed.

Transport Minister Phil Twyford has been fined $500 by the Civil Aviation Authorityafter boarding a plane and using his phone after the doors had closed.

Twyford has unreservedly apologised and will pay the fine.

He has now had his ministerial responsibility for the CAA reinstated, after Acting Prime Minister Winston Peters and Prime Minister Jacinda Ardern discussed the matter over the weekend.

Supplied Phil Twyford has been fined $500 for holding a phone call after a plane's doors had closed.

Twyford was caught in the act in May by an eagle-eyed member of the public, who then passed the information on to National's Housing spokeswoman Judith Collins.

READ MORE: Minister Phil Twyford apologises for Civil Aviation breach

Collins lodged a written question concerning the issue, after which Twyford to apologised and offered his resignation to the Prime Minister.

That offer was not accepted but his responsibility for the Civil Aviation Authority was handed over to Associate Transport Minister Julie Anne Genter.

Twyford wrote to the CAA to own up to the issue and said he made a phone call after the door has closed while his flight to Auckland was preparing for take-off.

"Mr Twyford stopped using his phone before the aircraft took off so his actions did not pose a significant risk to the flight," the CAA said in a statement.

"Nevertheless he did breach the Rule, has been issued an infringement notice and is required to pay a $500 infringement fine.

"It's imperative that passengers comply with Civil Aviation Rules and follow the instructions of cabin crew at all times."

Twyford said he would pay the fine on Monday afternoon.

"I reiterate my unreserved apology for using my phone after the aircraft doors had shut in preparation for take-off," Twyford said.

"This is inappropriate for anyone, but particularly inappropriate for me as Transport Minister.

"It is an important reminder to follow safety requirements around using phones on planes.

"I accept the findings of the investigation that I breached Civil Aviation Authority rules and will pay the $500 infringement fine."

When the issue first arose he said he was unaware of ever having broken the rules before.

Twyford said the call was about three minutes long, took place while the aircraft was taxiing, and was to a member of his staff.

Peters said he and Ardern had decided that given Twyford's contrition and the low-level of the offence it was appropriate his responsibility over the agency be reinstated.

He noted that it was a "wake-up call" to others who might have broken the rule on not using their phone before takeoff themselves.