Queensland's Environment Department had enough evidence to charge energy giant Santos over a massive oil spill but did not follow through, it has been revealed.

About 250,000 litres of crude oil leaked from Santos' Zeus oil field in Queensland's remote south-west, on the edge of the Lake Eyre basin, in May 2013.

A July 2014 department briefing note obtained by the ABC has revealed an investigation into the spill "determined that there was sufficient evidence to lay charges for breaches of the Environmental Protection Act 1994".

The document noted Santos has "historically had both major and minor spills ... which can be attributed to aging infrastructure and/or poor maintenance and management".

But Santos was not prosecuted.

The Wilderness Society's campaign manager Tim Seelig said that was the wrong decision.

"When a big oil and gas company does the wrong thing they should have the book thrown at them, they should feel the full weight of the law," Mr Seelig said.

"Because Santos wasn't prosecuted it didn't really suffer any major penalties, it was merely ordered to put in place provisions that should have been there in the first place."

"We will have more spills like this and we will have companies getting away with it."

In a statement, an Environment Department spokesperson said the spill did not harm the environment.

But they admitted the massive leak could have been significantly worse in the wet season, when floodwaters flush through the region into Lake Eyre.

The department said Santos was served with a "Restraint Order", meaning the company would do "significant work" at its own expense.

The LNP minister responsible at the time, Andrew Powell, said it was not his decision not to prosecute the company.

He said it was up to the department to investigate and make the final call in such cases.

A Santos spokesman said the company had "implemented a suite of design modifications and additional safeguards to prevent the same rare circumstances occurring again".

He said the area had been completely remediated and "a specialist environmental consultant assessed the spill site ... and determined there was negligible environmental impact".

But Santos did not offer more detail about the upgrade or how modifications at the oil field would prevent another spill.