Transport Minister Phil Twyford said the few minutes he spent on his phone while his plane was taxiing is the most expensive call he's ever made after he copped a $500 fine for breaching aviation rules.

The Civil Aviation Authority 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.

Today it announced he 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.

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"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."

Speaking to reporters this afternoon, Twyford was asked if it was the most expensive phone call he had ever made.

"Yes it is."

He estimated the call had been about three minutes long.

Twyford breached a rule which intended to prevent interference with instruments when the aircraft may be operating without visual references.

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He would not be drawn on whether the rule under which he was fined should be changed.

"I'm not going to look at that now."

Twyford had already apologised and offered his resignation to Prime Minister Jacinda Ardern over the incident.

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

Acting Prime Minister Winston Peters confirmed today that he had regained it.

Twyford said at the time 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 at the time that she expected all her ministers to act in accordance with the rules.

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.