Workers were today mopping up the remnants of the cooking oil and diesel spill which occurred down a channel in Bowen Park and into the lake, with most of the spill gone but the official all-clear not expected for several more days.

Environment Protection Authority director Daniel Walters said the authority was certain it was dealing with mostly cooking oil and some diesel but test results expected within a week would confirm the contaminants.

Exact quantities were unknown but believed to be "fairly small".

The EPA was conducting its own investigation into the spill, which started at the National Press Club in National Circuit and ran down Brisbane Avenue via the stormwater system. Other agencies might become involved in the investigation.

"We've chased the source of the spill up to some renovation works behind the National Press Club. We understand they've constructed a temporary kitchen facility so it would appear the spill has come as a result of those works being undertaken up there. So that's still under investigation as to what is happened to cause this incident," Mr Walters said.

"We've seen some cooking fats, basically like what you'd have at home, and some residual diesel, so there's been some involvement of a diesel tank that's up there as well. But the predominant source seems to be the cooking fats from the kitchen that's seems to have made its way down the stormwater system."

Mr Walters said there was no indication yet as to what had caused the spill or whether an individual or company would be prosecuted.

"For negligence under the Environment Protection Act, the penalty can be quite significant and it depends on whether it's an individual or company involved. So for a company, the fines are five times what it is for an individual," he said. "Once again that's a matter for investigation before we can make any further comment on that."

The spill was first reported on Tuesday morning with Mr Walters praising the quick response from the National Capital Authority, ACT Fire and Rescue, Roads ACT and Environment Protection Authority.

"Any incident that pollutes our lakes, the EPA always takes as a very serious incident. We move very quickly," he said.

"It's a credit basically to the NCA, Roads ACT, the EPA guys, who all worked very efficiently to put containment measures in place immediately and start the clean-up. So it was a very effective operation."

As well as floating booms, about 15 cubic metres of fresh soil along with hay bales and bidum material had been placed in the channel to prevent the oil and sediment going into the lake. The earth mound was expected to be removed today.

Clear skies had helped the recovery effort with a final assessment on the spill expected in the next couple of days to determine whether the other control measures such as the booms could be removed.

Bowen Park's famous swans appeared unharmed by the incident.

"There's been no evidence of any wildlife being affected by the incident either in the Bowen Park area or in the lake that we've been made aware of," Mr Walters said.