The tunnel, which links the pipeline route with Curtis Island, has been filled with sea water to buoy the 42- inch diameter pipe as it is pushed through.
It is expected to take up to four weeks for around 120 segments of pipeline, each measuring 36 metres in length, to be welded together and pushed through the tunnel using a large hydraulic jack.
The pipeline is expected to be connected to the LNG plant on Curtis Island in the coming weeks.
The 3.45-metre internal diameter tunnel is approximately eight metres below the sea bed. It was constructed using a 100-metre long, 277-tonne tunnel boring machine (TBM).
Since being launched in April 2013, the TBM averaged almost 140 metres per week, excavated 55,000 cubic metres of earth and lined the tunnel with 21,750 concrete segments.
The gas transmission pipeline is being built by Santos GLNG contractor Saipem Australia, and the tunnel was constructed by subcontractor Thiess.
Santos GLNG is a pioneering joint venture between Santos, PETRONAS, Total and KOGAS to supply liquefied natural gas to global markets.