Newly-independent MP Hone Harawira says it was "dumb" and shameful that he forgot to vote on the bill that is the main reason for his break from the Maori Party.
Mr Harawira quit the party last month amid disciplinary action taken against him over his outspoken criticism of the legislation that replaces the Foreshore and Seabed Act and his complaints about the Maori Party's relationship with the Government.
The Marine and Coastal Area (Takutai Moana) Bill, which is backed by National and the Maori Party, passed its second reading 62-56 yesterday, but Mr Harawira was not present to cast his vote.
In a statement late last night titled "Oops - forgot to vote", Mr Harawira said he was not present because he was distracted by business outside the house.
"I got caught up in a meeting outside, and completely forgot to vote," the statement said.
"Shame! Dumb! Doh!"
Mr Harawira said he had to be "a lot more focussed" now he was an independent, but added there was plenty of time to "fix it up" because the bill still has to go through its committee and third reading stages before becoming law.
Despite his oversight, Mr Harawira said his return to Parliament yesterday as an independent MP was "a real special day for me".
He said he put a lot of energy into his speech on the bill ahead of the vote.
"I was glad I got to lay down the views that I have been picking up on all round the country," he said.
Mr Harawira was able to speak on the bill from his new seat at the back of the debating chamber after the Labour Party gave him one of its slots.
He told the house the bill was "racist legislation and thousands will walk away from the Maori Party".
"With all my heart and soul I beg the Maori Party to recognise the fact that they have been sold down the river by the National Party and to accept the reality that they occupy a cold and lonely place in the hearts and minds of their people," he said.
Maori Party co-leader Tariana Turia said Mr Harawira's failure to vote "says something, doesn't it".
"You're either genuine and you're here to represent a particular viewpoint, and as such you're here to cast a vote saying you're here to support that viewpoint," she told Radio New Zealand.
Attorney-General Chris Finlayson, who is in charge of the bill, said that while restoring rights to Maori it also made it clear that no-one owned the common marine and coastal area.
"All New Zealanders have the right equally to walk, swim, fish, sail, dive, surf, picnic or play in it, just as they do now," he said.
Labour, Greens and ACT all opposed the bill.
Mr Harawira yesterday told media he was continuing to think about forming a new party.
"I haven't come up with anything solid yet, simply to say a lot of people are interested, a lot of organisations are keen, a lot of support is coming in."
Any new party would have to be Maori-led and Maori-focused, he said.
"I'm a Maori activist at the end of the day, and best I stay that way."