At 890km in length, the 42-inch diameter gas export pipeline is the longest subsea pipeline in the southern hemisphere.

The gas export pipeline will deliver gas from the Ichthys gas-condensate field in Western Australia's offshore Browse Basin to the onshore facilities at Blaydin Point near Darwin for processing.

"Completion of the offshore pipelay marks a significant milestone for the project," Ichthys project managing director Louis Bon said.

"It means we are one step closer to physically connecting our onshore plant near Darwin to the Ichthys field where our offshore facilities will be permanently moored for the 40-year life of the Project."

The pipeline has been installed by Saipem's SEMAC1 barge and Castorone pipelay vessel.

Inpex will now work towards putting first gas into the pipeline.

The milestone comes after the Japanese major warned that the cost of the project has blown out by at least 10% and start-up has been delated from the end of 2016 to the end of 2017, largely due to delays in the Asian shipyards that are building the key modules for the project.

The company has also been forced to extend its development drilling program after problems with the first development wells into the Brewster Formation.

Ichthys will produce 8.9 million tonnes of LNG and 1.6MMt of LPG annually, along with up to 100,000 barrels of condensate per day.

The project includes one of the world's biggest central processing facilities as well as one of the largest floating production, storage and offloading vessels.

Inpex owns 62.245% per cent of Inpex while France's Total is the other major partner with 30%.

CPC Corporation Taiwan (2.625%), Tokyo Gas (1.575%), Osaka Gas (1.2%), Kansai Electric (1.2%), Chubu Electric Power (0.735%) and Toho Gas (0.42%) all have minority interests.