From thick smoke blanketing the city to stifling weather that is breaking records, Canberrans have seen their share of unusual environmental phenomena this week.

Then a stormwater system turned green.

Jonah Lafferty stumbled across the creek in Canberra's inner-north this morning and was struck by its vivid colour.

"It looked bizarre. I had absolutely no clue as to why the creek was lime green," he said.

"It looked like perhaps a scene from when the Canberra Raiders were in 'the grannie'.

"I've lived in Canberra for a while. I'm a big fan of our stormwater system, but I've never seen it lime green."

The dye is non-toxic and non-pollutant. ( ABC News: Jake Evans )

The ACT Government, as it turns out, was behind the vivid change to the water in Sullivan's Creek in Lyneham.

A spokeswoman for the government said the Environmental Protection Authority had used the dye as part of a pollution investigation.

"The dye, Fluorescein, is non-toxic and causes no environmental harm," she said.

"It is expected that the dye will flow into the Lyneham pond and to be clear within a week."

Mr Lafferty said he was relieved to learn the creek was not in any danger.

"It was a pretty interesting spectacle to see the creek lime green," he said.

"But I'm surprised that they didn't put up any signs or anything just letting people know what was going on."