Adelaide City Council has been fined $90,000 for failing to cap a former landfill site to prevent harm to the environment, in what the sentencing judge described as a "deliberate" and "unacceptable" act.

The Council operated the Wingfield Waste and Recycling landfill site at Dry Creek from 1955 until 2004 for profit.

It was prosecuted in the Environment, Resources and Development Court for failing to cap the site by the required deadline in 2012 to prevent environmental harm.

Judge Susanne Cole found the council guilty of breaching the Environment Protection Act and imposed a financial penalty of $90,000 plus court costs.

"It needs to be made clear to the Council that, like every other natural or legal person in the State of South Australia, the Council is subject to the rule of law," Judge Cole said in her judgment.

"It is unacceptable for anyone, but particularly for a public authority, to simply choose to disobey an Act of Parliament on account of the expense that would thereby be incurred."

Judge Cole also noted that "no expression of contrition or remorse was offered".

"The offending was deliberate in the sense that the Council understood its obligations under its licence and did not appeal or object when the licence was issued.

"The Council was reminded of its obligations repeatedly by officers of the Environment Protection Authority, but did not undertake the capping required in accordance with the licence condition.

"This offending occurs in the factual context of much of the landfill remaining uncapped for nearly five years following closure."

The issue has been ongoing for years, with various capping plans approved in 2003 and 2008 on the four-hectare site.

"Nevertheless, instead of capping in accordance with one of those plans, the Council subsequently generated and submitted to the EPA several technically inferior and unacceptable capping plans," Judge Cole said.

The court heard that contaminants had been found along the landfill boundary within the upper and lower aquifer.

Lawyer for the Council, David Edwardson QC, told the court any penalty would have an impact on ratepayers.

Judge Cole said she had taken that into account, but also pointed out the "ratepayers of the Council benefitted from the conduct of the landfill for decades".

"I also note that the ratepayers of the Council are part of the South Australian community which was placed at risk of harm by the failure of the Council to cap the landfill in accordance with its licence."

The Environment Protection Authority's chief executive, Tony Circelli, welcomed the court's decision.

"This is a unique case and the first time in South Australia a metropolitan council has been convicted for serious breaches of the Environment Protection Act 1993," Mr Circelli said.

"This outcome sets a strong precedent and demonstrates the EPA will take action against any organisation or individual not complying with EPA requirements."

The Adelaide City Council no longer holds a licence for the site.