People for the Plains are arguing the approval for the company's Leewood wastewater treatment facility in the Pilliga forest is illegal, because it should not be classified as coal seam gas exploration.

The court case was originally scheduled for April 6, however it was adjourned due to new information and amended pleadings.

People for the Plains will be represented by the NSW Environmental Defenders Office (EDO) over the two day trial.

NSW EDO principal solicitor Sue Higginson said the Leewood facility was assessed and approved as a CSG exploration activity, but should have been subject to the development assessment process under the NSW planning system.

"In this way, Santos would have been required to prepare a full environmental impact statement, which would have been placed on public exhibition to allow the community the opportunity to scrutinise the proposal and its impacts," Ms Higginson said.

Santos general manager of energy NSW Peter Mitchley said the project was approved following a "rigorous and detailed" assessment process, which was carried out in accordance with relevant regulatory guidelines.

"The process included public consultation and submissions were received and taken into account in making the approval assessment," Mr Mitchley said,

"We worked through a number of government departments. All of the different government departments were satisfied with the sustainability and safety of the plant."

While People for the Plains is confident in its argument, it is concerned that if it does win the court case, the government may change the laws to suit Santos.

"We have seen this happen time and time again where local residents win a legal battle to stop an inappropriate development only to have the government or parliament change the rules," People for the Plains spokesperson Rohan Boehm.

"This happened to the community in Bulga, whose town faces decimation from an open-cut Rio Tinto coal mine, and it could also happen to Narrabri."

Fellow spokesperson Naomi Hodgson said should the group win, it will request Santos takes down the facility.

If you've built a shed without development consent council would make you pull it down, so why wouldn't the Government request Santos do the same," Ms Hodgson said.