Rodney Hide put his political credibility on the line last night by backing MP David Garrett after he admitted stealing a dead baby's identity to obtain a false passport.
And the Act leader admitted that he, too, had a conviction in his past.
Mr Hide said he knew about the 2005 identity-theft charges before Mr Garrett, Act's law and order spokesman, entered Parliament as a list MP, and he took full responsibility for the incident not being disclosed earlier.
Saying many MPs probably had undisclosed shameful incidents in their past, Mr Hide admitted he'd been convicted for drunkenness "when I was a very young man".
He told TVNZ's Close Up programme he had been arrested for being drunk and disorderly at London's Heathrow Airport when he was working on oil rigs. He said he was fined five pounds.
Earlier, he told the Herald he had "absolute" confidence in Mr Garrett and didn't believe he should stand down as an MP.
"I think the fact that he has made mistakes in his past makes him more credible."
Mr Hide retained his leadership of the party with the support of Mr Garrett and new deputy leader John Boscawen after a series of internal battles, including a messy stoush with his former deputy leader, Heather Roy.
In a personal statement to Parliament yesterday, Mr Garrett said that 26 years ago he had used a method he had read about in the spy thriller The Day of the Jackal to steal the identity of a dead baby and obtain a passport - an offence which can draw a maximum penalty of five years' jail or a fine of $15,000.
Mr Garrett said that at the time of the offence, he "naively" saw it as a "harmless prank", and he never used the passport. After it expired, he destroyed it.
But 21 years later, in 2005, he was arrested when police conducted a sweep of illicit passports after Israeli secret service agents were caught obtaining New Zealand passports using the same method.
Mr Garrett admitted the charges in the North Shore District Court and was granted permanent name suppression.
According to the court file, the judge told him: "There is no public interest in what you did 20 years ago."
The judge also said Mr Garrett had led a "blameless life", and reporting his crime would have consequences disproportionate to the crime that he committed.
Yesterday, Mr Garrett said he had apologised to the baby's family.
He has yet to talk to the media about the incident, and said he was uncertain about the extent of the suppression order's coverage.
Last night, Mr Hide told Close Up that Mr Garrett's offence was "horrific for the country for issues of citizenship and security, but it's horrific for the family and the mother that was affected".
He took "some responsibility, if not all responsibility", for the incident not being disclosed.
He had learned of it in initial discussions about Mr Garrett's standing for Act.
The passport identity theft and a Tongan assault incident were "the only two convictions" he knew about in Mr Garrett's past.
Meanwhile, Sensible Sentencing Trust head Garth McVicar, who introduced Mr Garrett to Mr Hide, yesterday confirmed he gave Mr Garrett a character reference at the court hearing on the identity theft charges.
Mr McVicar said the ideal opportunity for Mr Garrett to have disclosed the incidents would have been in his maiden speech to Parliament after the 2008 election.
"That would have killed it right there and then, rather than all this carry-on."
Yesterday's disclosure is also likely to undermine the credibility Mr Garrett earned among some voters with his criticism of the foreshore and seabed repeal bill which passed its first reading in Parliament yesterday.
Mr Garrett, who led the party's "three strikes" policy on tougher sentencing for serious violent offences, also admitted this week that he had been convicted of assault in 2002 in Tonga.
Labour deputy leader Annette King asked: "Is this strike two and is there a strike three?"
While the issue was quite old, "it's about the hypocrisy of taking a strong stand when you've got a past", Mrs King said.
Howard League for Penal Reform president Peter Williams, QC, who opposed Mr Garrett's three strikes legislation, said the Act MP had "demonstrated that no human being is perfect, and some recognition should be given to rehabilitation and repentance".
- additional reporting by Eddie Gay