Council was building a side track beside the damaged bridge on Wonwondah-Toolondo Road when the Mail-Times went to press last night.

Operations engineer Martin Duke said council was aiming to have the diversion available by late this morning.

"It will be suitable for all traffic, up to full B-double sized trucks, provided they travel slowly and carefully," he said.

"Traffic will be one way, controlled by traffic signals.

"We are placing message signs on the approaches to the bridge."

The existing bridge will be closed once the alternative is in place.

Until then, Mr Duke said a five-tonne gross vehicle mass load limit would apply.

"Cars should be okay, but trucks exceeding a five-tonne gross vehicle mass will not be permitted until the capacity of the structure is ascertained," he said.

An expert will inspect the bridge as soon as possible next week, with load testing to follow.

"We have found bad cracks on the deck structure of the bridge," Mr Duke said.

"The reason we had a look under the bridge was that it was due to have some repairs."

Engineers had been planning minor works on some edge beams when they discovered the major structural damage.

Council believes the damage occurred within the past six months.

"We inspected the bridge six months ago, and we did not record those cracks," Mr Duke said.

The bridge was built in 1957.