A new $5 billion rail tunnel to fix the growing congestion issues on Auckland's Harbour Bridge should be built within the next 20 years and before any additional roading network, according to a new business case.

The findings of the New Zealand Transport Agency business case, to be considered by Auckland Council's planning committee on Thursday, comes a month after Auckland was gridlocked in September when a truck blew over on the harbour bridge, damaging the structure.

But rather than adding additional lanes to the country's busiest route - the harbour bridge - or building a new road, transport officials recommend work be done on improving the public transport options.

Work is already underway to enhance the busway at a cost of between $500 and $600 million, and a detailed business case will be completed by the end of the year.

But within 15 to 20 years a new rail connection would also be needed at an estimated cost of $5 billion for a tunnel from the city centre to Takapuna and Smales Farm. A further $3 billion would be needed to extend the track from Smales Farm to Albany.

Transport experts did not believe a new road connection to ease congestion on the ageing Harbour Bridge would be needed until mid to late 2040 - 10 years after the rail tunnel.

The roading options would then be either expanding the capacity of the existing bridge or $10 billion for a road tunnel from the city centre to Esmonde Rd and additional motorway lanes to Constellation Drive.

In addition to the infrastructure costs of improving the connections between the North Shore and central Auckland, an additional $100-130 million annually for operations and maintenance would be needed, mostly for the management of tunnel operations.

Auckland mayor Phil Goff said you wouldn't be spending billions of dollars for a one in 60 year incident, which that truck blowing over the harbour bridge was, and the key factors were population growth and accessibility.

He said NZTA was "clearly leaning towards a public transit tunnel" as by the time the project would be completed Auckland would have a population of more than 2 million and there was not much point carrying more cars into an already congested city.

While both heavy and light rail was still be weighed up - Goff believed it was more likely to be light rail. If the popularity of the Northern Busway was anything to go by then people would use public transport, he said.

"If you are spending billions of dollars then you have to look across a harbour connection that will stand the test of time. Time and growth in population means that we have to look at more effective ways of getting people around the city rather than simple reliance on single occupancy cars."

Goff was grateful that most of the expensive infrastructure would be covered by central government.

He also believed a congestion tax would be another factor that could be considered to help manage the demand on the roads during peak times.