REMARKABLE engineering achievements in Gladstone these days seem to get ticked-off like small items on a checklist.

Here is another one. Engineers have simultaneously laid two, 1m-diameter gas pipelines across Gladstone Harbour.

The pipelines were winched 2.3km from the mainland to Curtis Island, which makes it Australia's longest large-diameter underwater "pipe pull".

The operation was completed safely over five days.

The pipelines are for the Queensland Curtis LNG and Australia Pacific LNG projects. QGC, operator of QCLNG, managed the project.

The two pipes were laid in the same trench by pipeline construction contractor MCJV, a joint venture between McConnell Dowell Constructors and Consolidated Contracting Company Australia.

The segments will connect each project's main pipelines from the Surat Basin gas fields, about 300km inland, to liquefied natural gas plants on Curtis Island.

This part of the overall LNG projects was no mean feat.

The harbour crossing involved construction of temporary facilities including a 2km-long road, two bridges and a railway line to move the pipes across two creeks, marshes and mudflats.

The harbour crossing used a temporary 450-tonne capacity winch on Curtis Island to pull the pipes, which have a combined weight of 8000 tonnes, through the sub-sea trench.

The trench will now be filled with gravel and rock to protect the pipelines.

QGC Pipelines Project director Norman Ingram said completion of the operation was a significant milestone.

"This engineering feat is significant on a world scale in terms of scope and complexity," Mr Ingram said.

"Its completion keeps us on track to become the world's first project to convert coal seam gas to liquefied natural gas."

The crossing follows the raising of the roof on Queensland's first LNG storage tank on February 8.