A 300-tonne boulder has fallen onto the Cunningham Highway south-west of Brisbane and blocked the road in both directions.

Police say the rock fall occurred near Gap Creek Bridge near Warwick and will leave the highway closed for at least 24 hours.

They say it is fortunate no cars were in the path of the boulder.

Major delays are expected overnight as the blockage is cleared.

Senior Constable Wayne Noffke says one lane should be open at some time on Wednesday.

"There is another rock that needs attention," he said.

"They cannot go near the rock that's on the road until that second rock is cleared so once it comes down, which will be tomorrow morning, they can then start working on the roadway."

Motorists are advised to seek alternate routes or delay their travel.

State Transport Minister Scott Emerson says it could be several days before the highway is completely reopened.

He says he was aware the rock face near the highway was unstable.

"We were trying to break it down and get it removed but unfortunately it has moved and landed on the highway," he said.

"So we'll assess it now over the next day to see how quickly we can remove it."