The partners in Australia's biggest coal seam gas project are expected to confirm plans to extend the venture within months after a Chinese company increased its investment.

Sinopec, a state-owned Chinese company, is pushing ahead with plans to tip $1.1 billion into the Australian Pacific liquefied natural gas development.

Origin Energy and its partners in the project are now expected to confirm a second LNG processing plant, with a final investment decision due within months.

The venture partners yesterday announced to investors a marketing supply agreement for a second "train" on top of the $14 billion start-up line for the plant, to be built on Curtis Island, southeast of Rockhampton.

It came as Sinopec, China's largest petrochemical company, signed off on plans to increase its purchase agreement for LNG produced at the site from 4.3 million tonnes a year to 7.6 million tonnes. The second production line is expected to cost about $6 billion.

The Australia Pacific LNG partners have signed a subscription agreement foreshadowed last month for Sinopec to raise its stake in the project from 15 per cent to 25 per cent.

Origin and US energy giant ConocoPhillips, the lead partners in the project, will have their stakes reduced to 37.5 per cent each. The deal must be approved by the Foreign Investment Review Board but Federal Resources Minister Martin Ferguson yesterday welcomed it.

It means another major LNG infrastructure project is a step closer in Australia.

There are 15 production lines being built across the nation to supply Asian markets in coming decades.

The Australia Pacific LNG partners had initially agreed to sell Sinopec 4.3 million tonnes a year, sourced from coal seam gas, for 20 years from mid-2015.

Sinopec's chairman Fu Chengyu said the project was an important part of the giant's energy portfolio.

Late last year, Japan's Kansai Electric Power Company also signed on for the sale and purchase of about 1 million tonnes of LNG a year for 20 years from 2016.

The deal comes after credit ratings agency Fitch last week warned of pressures on the cash flow of Australian oil and gas companies if they suffered cost overruns while building LNG infrastructure before the gas - and revenue - starts to flow.

Elsewhere, state-owned China Guangdong Nuclear Power Group is closer to bidding for Australian uranium explorer Extract Resources after upping its share in Extract's major stakeholder, Kalahari Minerals.