Transport Minister Phil Twyford has been fined $500 for using his mobile phone while his plane was taxiing, the Civil Aviation Authority has announced.

The CAA had been investigating Twyford's use of a mobile phone after the plane doors had closed and was taxiing for take-off during a flight from Wellington to Auckland in May.

Twyford had been issued with an infringement notice for breaching CAA rules relating to the use of portable electronic devices onboard.

"Mr Twyford stopped using his phone before the aircraft took off so his actions did not pose a significant risk to the flight. Nevertheless, he did breach the rule, has been issued with an infringement notice and is required to pay a $500 infringement fine," the CAA said in a statement.

"It's imperative that passengers comply with Civil Aviation rules and follow the instructions of cabin crew at all times," CAA director Graeme Harris said.

Twyford said today that he accepted the finding and would pay the fine.

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

Twyford had previously apologised and offered his resignation to Prime Minister Jacinda Ardern in relation to the incident.

READ MORE: Twyford offers resignation after airplane phone call

He lost responsibility for the Civil Aviation Authority as a result.

"I recognise that I made the call when I shouldn't have," he previously said in a statement.

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

Twyford said he was making a call to a staff member and thought it had been important at the time.

"In hindsight it doesn't excuse or justify breaking the rules."

Twyford said he had not given the matter a "moment's thought" until he received a written parliamentary question about it from National MP Judith Collins.

Ardern said she expected all her ministers to act in accordance with the rules.

READ MORE: PM confident Twyford phone saga has been dealt with

"As Transport Minister it is even more important that Phil abides by civil aviation laws," she said at the time.

"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."

It is not yet known whether Twyford will regain responsibility for the CAA following today's announcement.

National MP Gerry Brownlee was fined $2000 for breaching airport rules when he bypassed security to board a domestic flight in 2014 while he was Transport Minister.

Brownlee and two of his aides breached security at Christchurch Airport as they were running late for a flight.