Protesters climb onto a Lambton Quay ANZ branch to voice their opposition to the bank's fossil fuel investments.

Conservation group Forest & Bird has warned ANZ it could lose its business if it fails to divest from the fossil fuel industry.

Chief executive Hone McGregor wrote a letter to ANZ boss David Hisco​ delivering the ultimatum.

"We have given them a six month timeframe to develop a credible fossil fuel divestment plan," he said.

"If ANZ won't divest, we will re-evaluate our banking options with a view to transferring our business to a banking provider who has, or will."

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McGregor said climate change was a major threat to New Zealand's unique and threatened native species, and Forest & Bird was a strong advocate for reducing greenhouse gas emissions.

He encouraged other charities and businesses in New Zealand to deliver the same conditions to their own banking provider.

According to its latest accounts, the registered charity has annual gross income of $6.5m and the same amount of total equity.

ANZ has also come under attack from protest group 350 Aotearoa, which is making the same demands of the banking giant.

Protesters have blockaded ANZ branches in main cities around the country in recent weeks, forcing temporary closures.

While 350 Aotearoa and Forest & Bird are targeting change at ANZ New Zealand, the Australian banking group's local arm has few fossil fuel investments.

Spokesman Stefan Herrick said less than 0.27 per cent of total lending in New Zealand was to fossil fuels, and most of that was in gas.

The bank loaned much more to the renewable energy and green tech investment sectors, which were growing at about double the pace of non-renewables.

Herrick said ANZ New Zealand did not lend to coal-fired power generation or oil and gas exploration, and all its financing in power generation was for renewable energy.

The ANZ group's total lending to coal, oil and gas was about A$8.5 billion (NZ$9.15b), a number that had declining for the past five years.

Asked about Forest & Bird's ultimatum, Herrick said the bank did not comment on individual customers.