Community organisations, including unions and public health groups, are calling for an urgent independent review of the Trans-Pacific Partnership (TPP).

Farmers said the TPP would bring clear benefits for the agriculture sector, but others are concerned that Australia conceded too much in the deal.

Representatives from all 12 countries will formally sign off on the deal in New Zealand on Thursday, bringing the TPP one step closer to ratification.

But Dr Patricia Ranald from the Australian Fair Trade and Investment Network, together with 58 other community organisations, has written to Australian parliamentarians, urging them to reject the deal unless an independent assessment is done first.

Dr Ranald argues it "would be foolish for the Australian Parliament to rush" through its approval of the TPP and that there is plenty of time for the Productivity Commission to examine the detail.

The agreement is highly controversial in the United States, and it is likely the US Congress may not consider the TPP until the politically heated 2016 presidential election year is over.

Investor clause causes concern

Chief among the groups' concerns is the TPP's inclusion of an "investor-state dispute resolution" (ISDS) clause, which allows companies to sue governments in secretive tribunals if they believe government policy has hurt their commercial interests.

While ISDS clauses are commonly included in US agreements, Australia has generally shied away from them.

Former Prime Minister John Howard refused to allow an ISDS clause in the Australia-US free trade agreement signed in 2005.

The Productivity Commission, among others, has raised concern about ISDS clauses.

Tobacco manufacturer Philip Morris used an ISDS provision in an Australia-Hong Kong agreement to pursue the Federal Government over plain cigarette packaging after the company's High Court appeal failed.

The TPP explicitly states that tobacco companies can not take action under ISDS in the future.

Trade Minister Andrew Robb said the TPP also "carves out" environmental and health laws from ISDS challenges and he argues that will protect governments who legislate for environmental or public health reasons.

But Dr Ranald argues the explicit reference to tobacco control laws are the exception that prove the other carve-outs will not work.

Rural news in your inbox? Subscribe for the national headlines of the day.

"There's a clear carve-out for tobacco; governments can say there will be no cases on tobacco regulation.," she said.

"All of the other so-called safeguards are weak.

"They've been in various forms in other trade agreements and they haven't stopped global corporations from suing.

"In a way, the tobacco exception proves that, because to really exclude something, you'd need to do what they did with tobacco [explicitly state tobacco control laws cannot be challenged under ISDS], and they haven't done it for other health and environmental legislation."

Public health advocates speak out against Pacific deal

Australian public health advocates are also concerned about the agreement, and are unconvinced by Federal Government assurances that the Pharmaceutical Benefits Scheme (PBS) and the cost of medicines will be unaffected by the TPP.

A deadlock between Australia and the US over new-generation medicines called 'biologics' held up TPP negotiations for days before a compromise was reached in October last year.

Mr Robb is adamant that agreement protects Australia's current system and means there will be no change to the PBS.

He has said the "red line issue" was non-negotiable for him.

Michael Moore from the Public Health Association remains unconvinced that the compromise will work.

"They're going to be much more expensive drugs for Australians; it's going to hugely add to the costs — in the hundreds of millions of dollars — to the PBS," Mr Moore said.

"But when you go to Vietnam, when you go to Peru and countries like that, it'll mean they just can't afford these drugs.

"Actually, what it'll mean is that ordinary people who would be helped by these drugs simply won't be able to access them, and I think that's just an appalling outcome."

Deal-makers sign off on TPP

The signing ceremony in New Zealand on Thursday is partly an effort to maintain momentum towards ratification, but the path to get the deal off the ground is boggy.

The TPP is an Obama administration priority, but is generally hated by Democrats.

Some Republicans who favour trade agreement are nonetheless reluctant to hand a victory to the outgoing Democratic president.

The US is now in the throes of a presidential election campaign, and both Democratic candidates have distanced themselves from the deal.

There has been concern that the political environment will become too heated for the US Congress to sign off on a controversial trade deal this year.

If it is ratified, the US-led Pacific trade pact would cover 40 per cent of global GDP, bringing together 12 nations including Japan, New Zealand, Canada, Vietnam and Malaysia.

The TPP is the centrepiece of US foreign policy in the Asia-Pacific region and is seen largely as a vehicle to exert US influence and leadership in the region, balancing the growing power of China.

The deal has been controversial from the start, and World Bank modelling released in January argued that the group's developed nations, including Australia and the US, would see only a modest GDP improvement as a result of the deal.

Mr Robb rejected that analysis, saying the benefits of TPP should not be measured in GDP growth alone, but in its capacity to allow signatories like Australia to set the new global standards for trade, worker's rights and environmental expectations for the 21st century.

While Australian farmers were disappointed not to get more out of the United States for industries including sugar, beef and dairy, they did see significant concessions from Japan.

Industries such as Australia's wine sector see the TPP as their best chance to secure consistent trade rules across the region.

Australian farm advocates generally regard the TPP as essential in continuing to grow more profitable markets in the Asia-Pacific.