It has been described as a major milestone for the Tomago project and took place on Friday, April 11.

The 450-tonne dome was raised in three hours, from the floor of the partially-constructed LNG tank to the top, using two industrial fans.

A spokeswoman for the group said she believed it was the first time that a roof had been raised in this way in NSW for a similar type of structure.

Construction of the $310 million NGSF began in August 2012, and the site is expected to be fully operational by May next year.

The 1.5-petajoule tank will provide storage capability for up to two weeks of gas for the greater Newcastle region.

This can then be used as an gas supply during peak demand periods in the winter months.

AGL's NGSF construction project manager, Tim Knill, said it took hours of meticulous planning and preparation to raise the roof and the "operation went smoothly".

"The reason we raised the roof from the ground up, rather than build it at the top is because it's actually the safest and easiest way to do it," he said.

"Building it on the ground takes away the risk of suspending it in the air and working at heights.

"We had to factor in weather conditions before we could start but it went up without a hitch, which is a credit to the vast expertise we have working on this project.

"We raised the roof using two industrial fans.

"These fans raised the pressure under the dome roof sufficiently to lift it slowly into place. Once the roof reached the top of the tank, it was welded into position.

"It's a concept that sounds impossible to most people but our contractor has been using this method successfully since 1962," Mr Knill said.