An appeals court has ruled that the Environmental Protection Agency acted illegally last year when it suspended federal rules on toxic waste discharges in the first phase of the Reagan administration's campaign to reduce government regulation.

The U.S. Court of Appeals in Philadelphia said last week that the agency did not have "good cause" for failing to follow federal rules calling for public comment on proposed regulatory actions.

The suspended regulations govern the discharge of toxic chemicals such as cyanide, lead and nickel into municipal waste-treatment plants.

The court ordered the agency to reinstate the regulation retroactively to the date it should have taken effect before it was suspended--March 30, 1981.

The retroactive effective date will have a major impact on the electroplating industry, which now will be forced to comply with waste treatment rules by March 30, 1984.

The decision was a victory for the Natural Resources Defense Council Inc., which had sued the EPA.

The rule was one of 63 regulations President Reagan targeted for review less than two months after he took office, when his administration issued the first "regulatory hit list"--rules the administration considered excessively burdensome and costly to industry.

Two days after the release of the hit list, the EPA suspended the rule, saying the action was needed to comply with the president's order that called on all agencies to suspend or postpone all rules that had not yet taken effect, pending more detailed cost-benefit studies.

Dozens of other rules in 12 different agencies were suspended in similar fashion.

Administration officials said yesterday that they doubted other suspensions would be found illegal--primarily because agencies have called subsequently for public comments on rules they previously suspended.