Extending the life of the Springvale coal mine will raise to "extreme" the risk of damage to upland swamps while locking in long-term pollution issues for Sydney's water catchment, according to evidence prepared for the Land and Environment Court.

Whether the court will hear such evidence is in doubt, however, after the Berejiklian government revealed on Monday plans to weaken laws protecting water quality.

New legislation to be introduced to parliament on Tuesday will nullify a ruling by the Court of Appeal in August that the mine extension approval was invalid.

Opponents of the mine, which is located near Lithgow and is the sole source of fuel for the nearby Mount Piper power station, planned to call six experts later this month to challenge claims advanced by mine owner Centennial and plant owner EnergyAustralia to support the extension.