MP Gordon Copeland delayed his resignation from the United Future Party for two weeks to time it for the final vote of the anti-smacking bill - then missed the vote in Parliament last night.
"It's not a brilliant start, is it," he conceded when he realised he had been busy explaining himself outside the House when the vote was taken.
"I am very embarrassed."
The MP, who has broken with the party over its support for the bill, sought and gained the permission of the House to have his vote recorded retrospectively. The bill passed overwhelmingly, 113 votes to 8.
Government stability will not be threatened by the surprise resignation, because of its confidence and supply agreements with New Zealand First, United Future and the Greens.
While Mr Copeland's support for the Budget is not essential, he has agreed to support all legislation associated with today's Budget.
His vote on other legislation is unlikely to be crucial. Mr Copeland will remain in Parliament as an independent MP and at the same time revive as co-leader the Christian-based Future New Zealand Party that merged with Peter Dunne's United Party to form United Future.
Its main platform will be to repeal Sue Bradford's anti-smacking bill.
Mr Copeland said others in United Future, including board members, would resign over the smacking bill, which Mr Dunne supported.
Mr Copleand said yesterday that he made up his mind to resign two weeks ago, the day Prime Minister Helen Clark and National leader John Key announced their compromise on the bill. But he wanted to time it for the third and final reading.
Mr Dunne, who is also Revenue Minister, said he confronted Mr Copeland last week with rumours that he was preparing to leave and was reassured he was not. But Mr Copeland said he did not lie. He had reassured Mr Dunne only that he would not be setting up a party with independent MP Taito Phillip Field and Bishop Brian Tamaki of Destiny Church - a rumour that had been passed on to Mr Dunne by the Prime Minister.
Mr Copeland was able to deny that rumour.
"I have not lied to Peter at any point about the events of today," he said.
Helen Clark said yesterday that nothing had changed. "Life goes on exactly as is. Nothing changes. There is a majority on confidence."
It did not make things harder for the Government on non-confidence issues, she said.
After Mr Field resigned from Labour in February, Labour stepped up efforts to work across parties, including the Greens, Maori Party and the two confidence and supply partners, New Zealand First and United Future.
"This means the Government whip, Mr Barnett, will have a relationship with Mr Copeland very much like the one he has with Taito Phillip Field," said Helen Clark.
United Future deputy leader Judy Turner was distressed, saying she felt betrayed by Mr Copeland.
Mr Dunne was in Dunedin when he received a phone call from Mr Copeland giving him the news, which he received with "considerable surprise".
"I'm very sad. I'm very disappointed. I can't say I understand the decision but I appreciate his decision but that's life. You move on."
Mr Dunne was quite measured in his comments and dismissed Mr Copeland's reasoning that he was the one who was staying true to the party.
He did not think Mr Copeland, a list MP, had the moral authority to remain in Parliament.
If the party-hopping legislation had not expired at the 2005 election, Mr Copeland would have had to resign from Parliament if he resigned from his party.
* additional reporting Paula Oliver and Claire Trevett