Traffic on the Monaro Highway heading south is at a crawl after reports of a downed powerline near Hume.

The Google traffic map shows a dark red line on the Monaro Highway all the way south from where it crosses under Hindmarsh Drive to near Lanyon Drive.

The powerline was downed after a brutal storm swept through Canberra earlier this afternoon that saw a lightning strike start a fire in Gungahlin.