Greenpeace is using the contact details from people who sign petitions to cold-call them and ask for donations.

The tactic has been labelled invasive, unethical and a possible breach of privacy by a Victoria University marketing professor.

However, Greenpeace New Zealand fundraising director Michael Tritt said it would be "wrong" if it did not use the details people provided when they signed petitions.

"People aren't silly. They know if they put down a phone number or email, there will be some form of communication."

It is believed to be the first New Zealand charity to use the tactic, which it says is in response to shrinking takings from street collections as competition increases.

Mr Tritt supplied The Dominion Post with a copy of a petition against shark finning, which said in small print that it would use details "to keep you informed about ways to get involved with this campaign, as well as Greenpeace's other activities, funding etc".

Greenpeace would look at how this could be clearer, and petition collectors were trained to tell people that Greenpeace would "be in touch", he said.

According to Greenpeace NZ's 2012 annual report, its expenditure last year was $8.5 million. Of that, $2.7m was spent on fundraising.

Mr Tritt confirmed that was mostly to pay people to solicit money, but said for every dollar spent on fundraising, Greenpeace should get $3-$4 back.

It currently employs the equivalent of 50 fundraisers who get paid $18.40 an hour. They do not get commission but may get taken out for lunch if they had a good month, he said.

Of those, 20 were employed to call people at home, while 30 were "outreach campaigners" - street collectors sometimes uncharitably referred to as "chuggers", or "charity muggers".

Victoria University Marketing and International Business School head Professor Kim Fam said the new, more "invasive" technique went further than chugging.

"My question is: ‘How do I know you? What right have you got to [use my details]?

‘It's kind of a cold call, and you are invading my privacy'."

The fundraising industry is self-governed by the Public Fundraising Regulatory Association. General manager Karen Ward said it had started looking at this new type of fundraising.

When contact details were gathered, fundraisers had to give clear information on any follow up.

Very few complaints came through to the association about chugging, she said. "Our evidence shows that most New Zealanders like this type of fundraising. They like the one-on-one, personal approach."

Wellington City Council spokesman Richard MacLean said the council received a "steady stream of complaints about [chuggers], especially when they start blocking people's paths".

A spokesman for the Privacy Commissioner office said organisations collecting private information had to tell people how details were going to be used. If you give your details it was reasonable to assume they would be used to contact you.