Refueling mishap behind Swan River slick

A diesel slick on Perth's Swan River has dissipated with heavy rains overnight and appears to have been caused by a vessel refuelling mishap.

The Department of Environmental Regulation has not been able to pinpoint the exact cause of the slick, which spanned 500 metres in the Barrack Street Jetty area on Thursday.

After a downpour overnight, the slick has been diluted to a fine sheen on the water's surface.