The Labour Party says it can't rule out losing support after a website botch-up exposed its members' personal details online.
A database containing about 18,000 supporters' personal information could be freely downloaded from Labour's website until the problem was fixed over the weekend.
Blogger Cameron Slater obtained a copy of the data and has said he would name supporters on his website.
The Privacy Commissioner has raised concerns and is monitoring the situation.
Labour deputy leader Annette King today said the party had apologised to affected supporters.
"We have contacted as many people as possible to explain to them what happened, to alert them to what Cameron Slater wants to do, to apologise to them and to explain to them."
Ms King conceded the incident could hurt the party.
"I can't rule out anything mightn't hurt us at the ballot box, let's face it, that's political life.
"But I would hope that the people who have received the explanation from us will be satisfied with the actions that we've taken, and we moved very quickly to rectify the problem and to contact them."
Ms King said she hoped the incident did not turn people off supporting political parties.
She said Slater ought to be careful about releasing the personal details.
"He might want to hurt the Labour Party on behalf of the National Party, but it might be wise if he thought about other innocent people who are not expecting to see their personal information out in public," she said.
"I have to say I have no time for his style of politics and the sort of things that he's prepared to do."
Labour yesterday accused the National Party of downloading the data from its head office and tipping off Slater, calling it a "politically motivated attack".
National Party president Peter Goodfellow told NZPA a head office staffer had accessed the data but only out of concern that National's own website had similar vulnerabilities.
He said today National had written to Labour to say it had not passed on any information and did not intend to.
Mr Goodfellow said he had not personally seen the data.
"If somebody dropped it off to me I'd certainly look at it, but I'm not going to go out of my way to find out."
He said he had no idea about how Slater got the information and would not comment on "what any of the bloggers do".
Prime Minister John Key said he had no details about the incident and had not discussed it with Mr Goodfellow, who he arrived with at caucus today.
"I think Cameron Slater from what I've seen on his blog site...it's made quite clear that any information he had has not come from the National Party."
Mr Key said he had not had any discussions with Slater except on "the odd occasion when I've seen him at an event".
Slater said on his blog he had obtained a document showing Labour was using Parliamentary Services' resources for party business, which is against parliamentary rules.
Ms King denied that and said the party constantly reminded staff to be careful about such matters.
"I don't believe they've used them inappropriately outside the rules, but we will keep a very careful eye on what happened, just as every political party should do," she said.