Preparations are being made for another Waitemata Harbour crossing - with a tunnel east of the Auckland Harbour Bridge the Government's preferred option.

Transport Minister Simon Bridges this afternoon said he had asked the NZ Transport Agency to recommence work on what will be a "critical" transport link.

Steps would be taken to "future-proof" a proposed route, Mr Bridges said.

"The preferred route for the additional crossing is a tunnel east of the Auckland Harbour Bridge between the Esmonde Road interchange on the North Shore, and Victoria Park Tunnel and Central Motorway Junction in central Auckland," Mr Bridges said.

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"Advisors are preparing for the designation process and are putting together a business case focusing on the timing of construction and potential funding options," Mr Bridges says.

The Government has previously announced its support for a tunnel in preference to a bridge.

A business case will look at a range of public transport options, including heavy rail
An additional Waitemata Harbour crossing is likely to cost between $4 billion and $6 billion, and is likely to be needed between 2025 and 2030. Traffic growth and the rate of freight will be amongst factors that determine a construction start date.

Youth and climate change lobby group Generation Zero said it was disappointed with the decision to proceed with the second crossing.

"According to the government's own transport agency, with the success of the Northern Busway, traffic over the harbour bridge has completely flattened," spokesman Dr Sudhvir Singh said.

"The completion of the Waterview Connection, due to open in 2016, will give a full state highway alternative between Auckland and the North Shore, making an additional crossing redundant."

Generation Zero has advocated for its 15-year public transport plan, the Congestion Free Network, which is centred on the City Rail Link.

Dr Singh said the rail link would be able to move almost double the amount of morning traffic compared to the additional crossing.