Access across the river at Isa St crossing will be pedestrian and cyclist only until November while Mount Isa City Council builds a new bridge.

The new bridge will be 900mm higher and significantly wider than the existing structure.

The traffic lights on the intersection of Fourth Avenue and Isa Street will also be replaced with a roundabout, which will have three refuge pedestrian crossings.

Mayor Joyce McCulloch said the concrete support structures of the old Isa Street Bridge had been suffering from "cancer", further deteriorating their condition in the process.

"Work to demolish the existing bridge started recently, which will be followed by earthworks to raise the embankments on either side of the Leichhardt River and make the approaches to the bridge less steep," Cr McCulloch said.

Council says most of the construction work for the upgrade and bridge-replacement project will be done by local contractors, in a $7.5m project with $2.24m from the Federal Government's Bridges Renewal Program and $635,000 from the State Government's Transport Infrastructure Development Scheme and Council providing the $4.5m balance.

Deputy Mayor Phil Barwick said the new structure would have a higher flood immunity and increase carrying capacity from the existing five tonnes to be capable of carrying a loaded semi.

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