National Party leader Simon Bridges maintains that a system error led to people who signed petitions then being emailed and asked for donations.
People who signed up to join two petitions - against scrapping flights to Kāpiti Airport and one to save the Lumsden Maternity Centre in Southland - also started receiving emails from National asking for money to fight the government.
Mr Bridges said he understood that when people signed up, they became like party members.
"Instead of their emails being part of the petition process, they've been in the wider wash-up, which they shouldn't have. I can tell you all the advice that I've recieved this morning is that it was a genuine cock-up. We've apologised and need to make sure it doesn't happen again."
National would not say how many people were added to their database, but thousands signed the two petitions started by National MPs.
Neither petition stated that an email address would be added to the party's mailing list.
Joanne Dacombe, who signed the Kāpiti petition, said she felt her privacy had been breached.
"They risk alienating the very base they're trying to attract. It's a very unfortunate state of affairs for them.
"I think they were just a bit opportunist and thought 'that's it, we'll tap into this'," she said.
Ms Dacombe is now hesitant to sign any more National sponsored petitions in the future.
"Fair enough that I signed the petition, but I didn't sign up to receive National Party emails," she said.
Ms Dacombe wasn't the only one upset - many people expressed their disappointment online.
"Not impressed. I signed the petition to support a local project, not for my email address to be used for another political purpose," one person wrote.
Another woman said she "was seriously disgusted, & felt seriously insulted, when I received the begging letter from Simon Bridges".
Another person said the petition was "just a way to collect data to bombard your inbox with National Party propaganda".
The National Party said once it started receiving the complaints it realised email addresses had been mistakenly added to the database.
"We have since gone back to each individual concerned, apologised for the error, and removed them from the main email list," a spokesperson said.
"This is, and will always be, about championing local issues that matter to New Zealanders."
Political parties have been using different tactics for years to try and get people on to their email databases.
Two years ago, Labour had to defend its use of a "baby number" widget which offered people the chance to find out what number baby they are, in exchange for giving the party their email address.