Prime Minister John Key says ACT leader Rodney Hide has shown good judgment and has his full support following the resignation of disgraced MP David Garrett from the party.

Mr Garrett resigned today and has taken two weeks' leave from Parliament to consider whether he has a future as independent MP after revealing he had stolen the identity of a dead baby to obtain a false passport in 1984.

Mr Garrett confessed in the wake of a revelation he had an assault conviction in 2002 over a fight he had in Tonga with a doctor.

Mr Hide said he called Mr Garrett yesterday from Hong Kong, and told the MP he couldn't defend him because of an affidavit he had submitted to court in 2005 over the passport fraud case, in which he was discharged without conviction.

Questions have been raised as to whether Mr Garrett had included the assault conviction in the affidavit.

Mr Hide said the concerning aspect of the case was that in sentencing, the judge talked about Mr Garrett's previously blameless life.

Mr Hide was told by Mr Garrett about the passport fraud when he applied to be a party member, but Mr Hide said he considered it had been historic and was prepared to give him a chance.

Mr Key said in fairness, Mr Hide had acted on the information he had at the time.

"He was very much prepared to give Mr Garrett a go and in life people to get a second chance, but given the information that came to light he wasn't able to continue to support him.

"All I can say is that in the 22 months that Mr Hide has been a minister in my Government he's shown very good judgement and he has my full support."

Mr Key said he didn't know about Mr Garrett's previous convictions, and since the Act MP wasn't a minister, "I don't think I needed to know".

"I've got to say, as I said a couple of days ago, I find it extremely bizarre and odd behaviour."

Mr Key said he wasn't concerned about the stability of the government.

After losing Mr Garrett, ACT has four members, still giving the Government 62 votes - a majority of one.

"They'll continue to support us. I don't know how David Garrett would vote if he stayed on as an independent, what I can say is we wouldn't be looking to form any kind of relationship with him."

National and ACT signed an agreement after the last election, which guaranteed ACT would support the Government on confidence votes. This gave Mr key the ability to form the Government.

He subsequently reached a similar arrangement with the Maori Party.

When asked if whether he thought ACT will be in Parliament after the next election, Mr Key said "they've certainly had to deal with a number of quite destabilising events in the last few weeks.

"In the end it will come down to the New Zealand public and whether they choose to support them."