Protests are expected in more than 20 towns and cities today against a proposed trans-Pacific trade and investment treaty.

Critics of the Trans-Pacific Partnership Agreement (TPPA) said the treaty, planned for 12 countries including the USA, Japan, Singapore and Australia, was shrouded in secrecy and was a "corporate power grab."

Supporters of the TPPA, including New Zealand's Ministry of Foreign Affairs, said the deal would deepen economic ties and open up trade, boost investment flows, and promote closer economic and regulatory co-operation.

Protesters planned to gather in up to 23 centres, including all New Zealand's largest cities. Demonstrations in Auckland, Wellington and Hamilton were scheduled to start at 1pm and others were already underway.

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Christchurch event organiser Gen de Spa says the protests would be peaceful.

"There will be chanting and that kind of thing because we've got a message to get out there. But it'll be fun, it'll be a nice day out."

The Green Party said Kiwis were being sold a "dud" and there was too much secrecy around the TPPA.

"Information about the agreement has been made public only through leaks, despite repeated calls by New Zealanders to make the details public," said Green Party trade spokesman James Shaw.

"If the Government is so confident this deal is beneficial to New Zealand they should release the full cost-benefit analysis," he said.

"We aren't being told what benefits this deal will bring us... We know through leaks that the TPPA is a Bill of Rights for multi- national corporations."

Mr Shaw said the deal put the interests of major corporations ahead of New Zealanders' health, environmental and democratic rights.

"Make no mistake, this is a corporate power grab on a scale never before seen in human history," said Todd Rippon, Actors Equity New Zealand vice-president.

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"This treaty is so repugnant that if a full text of the negotiations were released to the public tomorrow it would cease to exist by the end of the week because anyone with a brain and a gag reflex would reject it outright."