Shell and Inpex are on the final stretch of a years-long race to export gas from offshore northern Australia, where both have spent billions of dollars building the world's biggest maritime vessels to grab a slice of Asia's booming LNG market.

Anglo-Dutch energy major Royal Dutch Shell and Inpex, Japan's biggest oil and gas producer, are vying for first gas from two overlapping fields after delays and cost overruns that have plagued both projects.

The pair have spent billions on offshore facilities, including Shell's 490 m (1,600 ft) long Prelude floating liquefied natural gas unit and Inpex's Ichthys Explorer semi-submersible platform, both the world's largest of their class.

"Prelude and Ichthys are both record breaking megaprojects that have been competing throughout their development," said Saul Kavonic or energy consultancy Wood Mackenzie.

Both arrived on site last year and are about to receive what's known as a "cooling cargo" of liquefied natural gas (LNG), which indicates a plant is getting closer to starting output.

The delivery is used to chill the facilities ahead of regular production of LNG, which is shipped at a temperature of minus 160 degrees Celsius.

The race means more than just pride as the Prelude and Ichthys fields straddle the same gas in the Browse Basin, according to WoodMac.

"Their reservoirs are connected, meaning whichever project starts producing first will draw somegas away from the other project," Kavonic said.

Ship tracking data in Thomson Reuters Eikon shows the Gallina LNG tanker arrived at Prelude in mid-April in order to deliver a cooling cargo, while the 'Pacific Breeze' is due to arrive at the Ichthys field from Malaysia this week.

Both projects are still preparing for export, but Shell is grappling with the challenges of commissioning a brand new technology.

"We currently assume first cargo from Ichthys in Q3 and from Prelude in Q4, with risk of further delays," said Kavonic.

A Shell spokeswoman reiterated that the oil major expects to get cash flow from Prelude in 2018.

Inpex has said it expects a start-up at Ichthys, initially due in 2016, in April or May.

"There is no change to the schedule that we announced in March," Inpex spokesman Carlo Niederberger told Reuters.

Although both projects feed from the Browse Basin, Inpex said it was not yet certain whether there would be direct competition for gas between the two fields.