Queensland Gas Company Ltd (QGC) has increased the gas reserves required for a proposed $8 billion liquefied natural gas (LNG) development with BG Group Plc.

The coal seam gas (CSG) producer has increased its proved (1P) reserves by 16 percent to 705 petajoules (PJ) and its proven and probable (2P) reserves by 12 percent to 2,703 PJ.

The company is targeting more than 7,000 PJ of 2P gas reserves to support a planned LNG plant at Gladstone on the Queensland coast.

"QGC's concerted exploration and development campaign will continue apace as we position the company for our domestic strategy, revolving around gas-fired power stations, and our international strategy with BG Group and the Queensland Curtis LNG Project," managing director Richard Cottee said in a statement.

QGC and BG Group, the UK's third largest natural gas producer, are one of four group's aiming to develop separate LNG plants - using coal seam gas as feed - for export to lucrative Asian markets.

Shares in the company dropped three cents to $4.17 by 1607 AEST.

The increase in 1P and 2P gas reserves came after an analysis of production data and final confirmation by independent certifiers Netherland, Sewell & Associates.

QGC said its proved, probable and possible (3P) reserves decreased from 7,163 PJ to 7,103 PJ after the analysis.

Meanwhile, QGC said the sale of 1.2 million shares by Mr Cottee last week was the result of a "significant" tax bill received by the company's managing director and not a margin call.

QGC said the "tax liability, due in late September, arose from an assessment of the tax payable on QGC shares held by Mr Cottee", who has retained a direct interest in about 4.9 million shares.

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