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In a message displayed at protectmarriage.org.nz, 24/7 Hosting & Web Design said: "Due to large scale Denial of Service attacks against this domain it has been decided to ensure the stability and security of our servers and network this account has been removed."
Conservative lobby group Family First this announced the launch of site this morning.
It came after Labour MP Louisa Wall's members bill to redefine marriage, which is not currently defined in the Marriage Act, was last week pulled from the ballot.
The bill would make it clear that two people, regardless of their sex, sexual orientation, or gender identity, could to marry.
Family First national director Bob McCoskrie said his group had launched the website to protect the current definition of marriage, which he described as "one man, one woman''.
The website featured an online petition to Parliament and a tool to let people contact MPs to express their views.
But by midday the site had crashed.
Mr McCoskrie contacted the web host company which said the site was "experiencing a large scale denial of service attack''.
A denial of service attack is commonly achieved by saturating the targeted website with external communications requests, leading to a server overload.
By 2pm the website was working again, but a notice on 24/7's site this afternoon said "large scale" attacks had continued throughout the day.
Mr McCoskrie was told it was a fairly major attack, which was aimed at the protectmarriage website but also took down quite a few of the host's other websites.
He did not know where the attacks were coming from.
"You always hope you can have a robust debate about ideas, and show respect for each other but when you're trying to take out each other's website it kind of suggests that you're not going to get a good debate, so that's disappointing.''
In explaining the website this morning, Mr McCoskrie said politicians had been hammered recently with reasons to redefine marriage, and the website would help to balance the debate.
"Ultimately, the state - which did not invent marriage - has no authority to re-invent it,'' he said.
"Equality does not mean we must redefine marriage. Same-sex couples have the option of civil unions to recognise their relationship so there is no need for redefining marriage.''
Adding to the website's early-launch woes, US band Train vowed to get one of their songs removed from the website, after a YouTube link to the song Marry Me was placed on the website without them knowing.
Train was asked yesterday by a New Zealand tweeter, Mikey-J-S, why their music video appeared on the website.
Train responded: "Didn't know. Getting it off asap. Tnx 4 tip''.
Mr McCoskrie said he had not yet heard from Train, and the song remained on the website.
"We're not going to do anything based on Twitter, but if they contact us and ask us to remove, we will certainly respect their wishes.''
Initial indications are that the bill has the numbers to pass. Of the 76 of Parliament's 122 MPs who responded to a New Zealand Herald survey last week, a clear majority of 43 were in support of the bill or leaning towards backing it.
Labour leader David Shearer has said he will support the bill, and Prime Minister John Key today said he would vote in favour the bill.