A statement said the cancelled flight was the last of the evening and there was no opportunity to transfer affected passengers to Auckland
The diverted aircraft had spare seats for seven passengers including the Prime Minister and his entourage.
"Air New Zealand does occasionally divert flights to pick up passengers from cancelled flights if there is limited or no opportunity to re accommodate those passengers onto other services directly from that airport."
Passengers were informed about the diversion before the flight, the statement said.
Ms Bradford said a friend had told her about the flight.
"Last night she received a call from an irate Lee Heller, who had been a passenger on direct flight Air NZ 8388 from Nelson to Auckland.
"Out of the blue, passengers on that flight were advised that the plane was to be diverted to New Plymouth for 'operational' reasons.
"Little did they realise that 'operational' meant their flight was diverted simply to pick up the Prime Minister."
The flight was over half an hour late as a result, Ms Bradford said.
"On top of that, while in the air the stewardess announced that passengers should vote National, apparently at the request of John Key.
"This goes beyond the sort of born to rule arrogance that is normal with National.
"These are the actions of a man who has lost all perspective on how a Prime Minister should behave."
Victoria Davis, a Golden Bay resident who spoke to Heller, said passengers were initially afraid when they heard the plane was being diverted for an "operational matter".
That fear turned to anger when a flight attendant told Ms Heller the plane was picking up Mr Key, Ms Davis said.
"The people sitting next to her were really upset because they were late and they couldn't believe it was happening.
"People were really nervous and upset. If you are on a non-stop flight you expect it to go straight there unless there's a damn good reason."
Meanwhile Air New Zealand said an in-flight announcement where a flight attendant told passengers Mr Key wanted them to vote National was a response to "light hearted banter".
"While on their way to Auckland the flight attendant responded to some light hearted banter amongst the passengers and at the end of her PA announcement reminded everyone to vote on the weekend and mentioned John Key wanted her to vote for him."