A large barge being used for underwater drilling in southern Lake Macquarie is expected to be on the water for the next three months.
Centennial's rockhead drilling program began a week ago to help determine the distance between the workings of its Myuna Colliery and the lake floor.
The 18 x 9.3 metre barge has been spotted off Murrays Beach in recent days and will operate between Wangi Wangi Point, Swansea, Pulbah Island and Murrays Beach over coming months.
The drilling program, which followed a controversial seismic survey, is being completed in stages.
The first stage involves drilling 17 boreholes over 20 days using a small truck-mounted exploration rig on the back of the barge.
Work is being undertaken between 7am and 6pm, approximately five-and-a-half days per week.
There is a 50-metre exclusion zone around the barge, which features navigation lights for the safety of lake users during darkness.
Centennial said the "jack-up barge" was "carefully chosen for safety, stability, time efficiency and to minimise any potential environmental impacts".
"This type of drilling is used to determine rockhead thickness and to calibrate the sparker survey sub-bottom profiling survey results," the company said.
"For each of the 17 boreholes, the small 100 millimetre diameter cutting head will drill into the lake floor through the sediment until [one] metre of solid rock is found, after which the cutting head is extracted and the barge moves onto the next borehole location.
"After analysis, testing and updating of the geological model, a further drilling programme will be scheduled."
A review of environmental factors (REF) prepared for Centennial last year by environmental consultants GHD said the drilling would take place over four 16-week periods over the next five years.
All up, 60 surface-to-seam boreholes will be drilled.
Centennial came under fire in late January when details of the works emerged.
It delayed its seismic survey by a week after an outcry from environmentalists and fishers about the potential impacts on marine life, but it ultimately went ahead.
Despite suggestions of an on-water protest by those opposing the work, the seismic survey and drilling program proceeded as planned.
Centennial says it completed similar survey work in 2009 and 2010.
It says the work is essential to ensure a safe distance is maintained between the lake floor and the Myuna Colliery mine workings.
The colliery has been mining under Lake Macquarie for about 40 years.
The coal from Myuna is supplied direct to Eraring power station via an overland conveyor.
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