Calls for greater openness over a $1.2 billion coal mine proposed for the Liverpool Plains are mounting after the Berejiklian government admitted it had taken more than two years to make a key decision on the project's future.

China-owned Shenhua applied in January 2016 to have suspended a condition in its exploration licence that required it to make "substantial" progress of the Watermark mine within eight years, according to a document obtained under freedom of information laws by Lock the Gate.

Protesters marched in Sydney on March 24 to protect farmland, water and rural communities from coal and coal seam gas, including the Shenhua mine. Credit:Kate Ausburn

That eight-year period expired the following month.

Despite that deadline passing without major mine work, the government controversially refunded Shenhua $262 million in July 2017 in return for surrendering about half its exploration licence area.