An immediate 15-tonne load limit has been placed on the Namoi River Bridge on Manilla Road.
Damage to some of the bridge's support structures was found last week during regular maintenance work.
While repairs are carried out, the load limit will be in place.
A signposted detour is in place for all vehicles weighing over 15 tonnes, adding between five and 10 minutes to most journeys.
The detour will be via South Street, River Street, Higgins Avenue, McDonald Street, Rangari Road and Charles Street.
Repairs are expected to be complete by the end of May, with community and stakeholders kept informed when the load limit is removed.
Transport for NSW apologises for the late notice, and thanks motorists for their patience during this time.
Motorists are advised to drive to the conditions, and follow directions of signs and traffic control.