Key described the hair-pulling as just "horsing around", as he had a "fun relationship" with staff at Rosie cafe near his home in Parnell, Auckland.

The prime minister has admitted he misread the situation when he pulled a waitress' ponytail over a period of months, but denies it was a sexist act, saying he "could have" done it to a man.

Tuesday marks Key's first day back in Parliament since the news of the incident broke - he was travelling to WWI centenary commemorations in Gallipoli at the time.

Key apologised to Auckland waitress Amanda Bailey, 26, after it emerged he had repeatedly pulled her hair while she was serving him over the course of months.

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He described the hair-pulling as just "horsing around", as he had a "fun relationship" with staff at Rosie cafe near his home in Parnell, Auckland.

On Monday Key denied the ponytail-pulling was sexist.

Asked if he would have done it to a man, he responded: "I could have."

"Well there's a bit of context here, and the context was a very good-natured environment we were in and it was very much a sort of thing in jest, so I guess technically it would have been possible," Key said on Radio New Zealand's Morning Report.

The Prime Minister said "I don't think so, from memory, no," when asked if he pulled the hair of other people he met.

Key admitted he had "misread" the situation.

"I can really assure people it was really meant in good humour and nothing else."

"There was some context there in the wider environment but every person on the planet would accept that I should have read that situation better and I didn't, and I regret that and I've apologised sincerely for it."

Key acknowledged there was media footage circulating of him touching other people's, including children's, hair, but said: "I haven't watched it."

"It's a sort of fairly tactile world we live in, you know people come up to me all the time - I don't know how many thousands of selfies I do in a week or a month but you know, people put their arms around me, lots of kids come up to me, all that sort of thing.

"But if you're asking if it's an issue, I don't believe it is, no."

Key told Radio New Zealand being casual around people in wanting to make them feel comfortable was a strength of his, but it could also be a weakness.

"Clearly, you look at it now and you look at it written down and you say clearly it really just doesn't read at all well, and I'm the first to accept that.

"So that behaviour has to change - it will change, and I accept that fully."

Bailey has called in Unite Union to represent her in any upcoming action she may take against Key, or her employers at Rosie cafe.

Key did not have a view on whether Bailey's actions were politically motivated.

Private prosecutor Graeme McCready has filed papers in the Auckland District Court to lay a charge of "male assaults female" against Key.

But Key refused to say whether he would step down as Prime Minister if charges were laid, saying there were a lot of steps to be taken before that situation arose.

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