Drivers were being warned to expect long delays on the Monaro Highway near Royalla after a crash blocked both lanes on Monday afternoon.

An ACT Policing spokesman said a single vehicle towing a trailer had rolled about 3.20pm.

"Firies are on the scene trying to get someone out of the vehicle," the spokesman said.

"Hopefully we'll have the traffic moving again before too long. It doesn't look like there are serious injuries, thankfully."

On Sunday the highway was blocked for several hours following a multi-car crash just past the ACT border at Williamsdale.