Transport Minister Stephen Joyce says building will need to start on a new harbour crossing between 10 and 20 years from now.
The New Zealand Transport Agency on Wednesday approved an extra $41 million to complete strengthening work on the Auckland Harbour Bridge and began to protect the route for an additional harbour crossing.
It 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, estimated to cost $3-4 billion.
Mr Joyce told Morning Report the work that's being done now to repair the harbour bridge will extend its life in its current configuration for about another 15 - 20 years, and, with more restrictions on how it is used, for 40 years.
The completion of the western ring route is the most urgent matter in Auckland, he says, and construction on a new harbour crossing will need to start about 10 and 20 years out.
North Shore mayor Andrew Williams says the tunnels will be best for the city's long term future.
However the ANZAC Centenary Bridge campaign group says a new bridge would be more cost-effective and better for the environment.
Auckland City mayor John Banks does not discount replacing the bridge, but says there are more barriers to that than a tunnel.
Meanwhile, Mr Joyce said he would have liked the New Zealand Transport Agency to have told him earlier about the cost overrun for bridge repairs, though he accepts that in this type of project costs are not fully known until work is underway.
Strengthening work on the bridge's clip-on lanes began last year with a forecast cost of $45 million.