Sticking points in the tortuous negotiations - that were originally set for just two days - over market access for agricultural exports such as sugar and the contentious area of clinical data exclusivity governing biologic drugs were described by those close to the exhaustive process as "nearly resolved".

The complex 12-country deal, billed by Trade Minister Andrew Robb as the world's biggest trade liberalisation in two decades, has the potential to reduce or remove 98 per cent of import tariffs on Australia's products being sold into the Pacific region.

Sugar, long the most intractable of issues in discussions with the US, may get twice as much access to the giant US market, while beef is expected to be among the biggest winners. Other producers will also benefit in areas as diverse as wine, horticulture, grain, dairy and seafood, which are expected to enjoy new tariff-free access along with a raft of professional services.

With last-minute hitches always possible, weary negotiators were nonetheless optimistic of making the historic announcement at a triumphant press conference scheduled in Atlanta, Georgia, for the early hours of Tuesday Australia time.

It had already been delayed once.

Government sources said Mr Robb had held firm against intense pressure to lengthen Australia's clinical data exclusivity protections in line with the US, in order to shield pharmaceutical companies from competition at the hands of generic producers who had not carried the costs of drug development, testing, and subsequent innovation.

Australia had led a group of countries including Chile and Peru in holding out on this issue in order to contain the burgeoning costs of subsidising a vast array of drugs listed on the Pharmaceutical Benefits Scheme.

Pressure for an Australian capitulation included a telephone call from US President Barack Obama to Prime Minister Malcolm Turnbull last Friday.

Mr Robb said the deal was "shaping as a very strong package for Australia in areas such as agriculture and services".

Fairfax Media understands the US eventually backed down from its insistence on an eight-year fixed period of data protection for biologic drugs, instead staying with five years.

In the case of some drugs such as those used in leading-edge cancer treatment, the US offers originator exclusivity of 12 years in recognition of risk and to encourage ongoing investment in research.

Another key sticking point for health and environment advocates was the inclusion of a so-called investor-state-dispute-settlement mechanism, known in the trade as ISDS.

This facility, which is common to many bilateral free trade deals including the ChAFTA, allows foreign companies to sue sovereign governments over laws they claim have disadvantaged or entirely denied entry of their products in those markets. A prominent contemporary example is cigarette companies suing the Australian taxpayer over Canberra's switch to plain-paper packaging.

But it is understood Mr Robb has succeeded in including "safeguards" against vexatious ISDS-facilitated claims where local laws have been implemented in the interests of public health.

The full details of the biologics plan are still unclear, but it is believed to maintain the status quo for Australia, which currently provides brand-name drug manufacturers with five years' protection of clinical trial data that is submitted to regulatory agencies for approval. This means manufacturers of cheaper follow-on versions of the product cannot rely on the clinical trial data produced by the originator of the drug.

The US has been pushing for at least eight years, which has worried public health experts in Australia who say even one extra year of data exclusivity could cost the Australian government hundreds of millions of dollars in extra costs for the Pharmaceutical Benefits Scheme.

Biologics are medicines made from biological sources. They include vaccines such as the cervical cancer vaccine Gardasil and cancer drugs such as Keytruda, a melanoma drug recently listed on the PBS. Without the PBS subsidy, Keytruda would cost more than $150,000 for one patient per year.

Researcher and spokesperson for the Public Health Association of Australia Deborah Gleeson said the new agreement appeared to include two options: one of eight years' data exclusivity for some countries, such as Canada and Japan where that is the current term, and one of five years' with a "vaguely worded clause" for Australia to keep its current arrangements.

"But as with all these things, the devil is in the detail and so it's very difficult to evaluate whether Australia's system would be adequately protected and also what it would mean for other countries," she said.

Dr Gleeson, of LaTrobe University, said it was also unclear if the deal would lock in Australia's current intellectual property arrangements for drugs, which were criticised by a government review in 2012-13 and are currently being examined by the Productivity Commission.

"The previous government did a review of our pharmaceutical patent laws in 2012-13 that recommended we actually wind back the monopoly protections on pharmaceuticals. It was very critical of Australia's intellectual property system and of the trade agreements that Australia had signed up to," she said.

"That report was never acted on and now we have another opportunity to review our arrangements and potentially make changes to ensure we have affordable access to medicines in future, especially with new, very expensive medicines coming on to the market, many of which cost tens of thousands or hundreds of thousands per patient per year."

The TPP has been a long-term priority for President Obama, and is regarded as one of the key foreign and economic policy reforms he regards as central to his term in office.

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