Future investment in Australian coal mining projects is being placed at risk if governments continue to ignore the impact of high costs, delays and uncertainties in the approval process for mining projects, Australian Coal Association chief executive Nikki Williams warned.

She said the decision by the NSW Land and Environment Court's decision to reject the Rio Tinto Coal & Allied's Warkworth extension project was the first time a court had overturned a major project approval to extend an existing open cut mining operation.

"The decision has set a dangerous precedent because it means that after a company has gone through three and a half years of extensive planning processes and finally receive government approval, the project can still be rejected," she said.

"How can any project proponent proceed with a new project if there is no end point in the planning process?

"Project proponents must have clear direction about the process that will apply to their proposal and the government requirements that have to be met. All of that has gone out the window."

Rio claims hundreds of jobs at its Warkworth complex may be lost if an appeal against a Land and Environment Court decision to halt its planned expansion of the mine is not expedited by the Supreme Court.

In an affidavit to the Supreme Court, Rio Tinto subsidiary Coal & Allied argued that the Warkworth project – which aims to maintain production levels of 18 million tonnes per annum and extend operation of the mine beyond 2021 – needs an early appeal decision by the court to provide certainty.

“At extraction levels less than 18 million tonnes per annum (which provides approximately 12 million tonnes of product coal per annum) the economic viability of Mt Thorley Mine Complex can be threatened in certain economic situations,” the affidavit states.

“Further significant job losses are expected at the mine if economic conditions remain the same as present unless the mine can maintain its volume of product coal produced at 12 million tonnes per annum.”

Williams said Australia trades in a global coal market and it must keep its costs and its risks down, particularly at a time of difficult market conditions.

"Coal companies must have certainty in the planning process if they're to secure investment and remain globally competitive," she said.

"This is the commercial reality: uncertainty and delays cost money, and they are now costing money at a time when the Australian industry is facing pressure from low prices, high costs and a high Australian dollar.

"Governments should be ensuring certainty in regulations and keeping costs down. We cannot take the revenue from Australia's successful coal export industry for granted."