The cost of upgrading the Auckland Harbour Bridge has almost doubled with the New Zealand Transport Agency approving an extra $41 million to complete strengthening work.
The agency also says the bridge has a limited lifetime of 20-40 years, so work has begun to protect the route for an additional harbour crossing.
Strengthening work on the bridge's clip-on lanes began last year with a forecast cost of $45 million.
The agency's State Highways Manager, Tommy Parker, says the scope and complexity of the work means more labour hours and more than 40% more steel are now required.
He says an engineering report shows ways to increase the bridge's lifespan, but a new crossing will eventually be required.
Mr Parker says the agency has lodged notices of requirement with the Auckland City and North Shore City Councils to ensure land remains free for the option of building two road and two rail tunnels.
"We need to start to start planning for a future crossing - an additional crossing - before the life of the clip-ons expires, so that's why we're moving to protect the route today."
Mr Parker say the planning process for the tunnels, estimated to cost $3-4 billion, could take about 20 years.