The dome-shaped roof was raised about 40m by little more than air.
GLNG downstream vice-president Rod Duke said the roof was lifted by electric fans, using the same pressure needed to blow bubbles through a straw in a glass of water.
"We used air to raise this huge roof, around 40m inside the outer concrete walls of the LNG tank," he said.
"We can now start to build the inner nickel steel tank that will hold the LNG at -162C."
The roof will be covered by two concrete layers.
Construction of the project, which shares its home on Curtis Island with two other large-scale LNG projects, is past the halfway mark, said Santos GLNG vice-president Trevor Brown, who delivered a project update at this year's APPEA conference.
He said first cargo was expected in 2015.
The Santos GLNG project will convert CSG into LNG. It will be stored in the tanks ready for off-take by vessels entering Gladstone Harbour.
There are two tanks at the GLNG project, with each train able to fill a tank within a week, depending on the frequency of cargo vessels coming in to export.