Koalas are smarter than experts predicted and will cross highways using "tunnels and bridges" to access bushland.

Professor Darryl Jones, a Griffith University ecologist, found he had to "eat his words" after research showed that within three weeks koalas were regularly using the bridges and tunnels built for them.

"We found that if we put in the structures and put in the fences – so they could not get on the road – then they had to seek other ways to get from A to B," Professor Jones said.

"I thought these are not an intelligent animal, they spend their time high on eucalyptus oil and you would not call them innovative," Professor Jones said.