The NZ Transport Agency (NZTA) made the announcement yesterday.
Work would include seismic strengthening of the 29 pier caps and two bridge abutments to get the infrastructure of the bridge up to earthquake code.
An NZTA spokeswoman said the pier caps were designed to evenly distribute the load of the bridge deck and traffic on it over the piers.
The bridge deck is the part of the bridge where traffic flows.
The piers are the part of the bridge structure between the abutments that hold up the deck and normally drilled into the river bed.
Fulton Hogan Ltd was awarded a $2.5million contract to strengthen the bridge.
When preparation for the bridge strengthening work starts, the 100kmh SH1 zone will be reduced to 30kmh while lane closures were in place.
When workers are on site, speed on the bridge is expected to be 70kmh.
The work is expected to be completed later this year.
