Work on the $10 billion transport package for Auckland could start sooner than 2020 if employment in the central business district and rail use rise faster than expected.
The Government has announced funding for an infrastructure upgrade including a rail loop, a tunnel under the Waitemata Harbour and a series of state highway improvements.
The plan would see construction of the rail link start in 2020, several years later than Auckland Mayor Len Brown had been pushing for.
This was despite the Government ''broadly'' agreeing with last year's Auckland Transport's City Centre Future Access Study, which concluded that the best way to meet the city's needs would be the city rail link and upgrades to the bus network.
''I realise 2020 is not what the [Auckland] council is wanting, but while we may differ on timeframes, there is clear recognition by the Government that the project will be needed to address access to the Auckland CBD and improve the efficiency of rail,'' Prime Minister John Key said.
''We don't, however, agree with the timeframes proposed in the report, which concluded that the City Rail Link needs to be in place by 2021,'' he said.
''Given the scale of the project, this would effectively mean construction would need to start in two years' time.''
Key did however comment that if two conditions were met, the work could start earlier than 2020.
These were a 25 per cent rise in city centre employment, and for annual rail patronage to hit 20 million trips a year.
Prior to the rail loop construction, Key acknowledged that bus crowding and congestion coming into the CBD was ''a priority issue'' and the Government would make funding available in the next transport policy statement to address it.
The alignment of the second harbour crossing was yet to be determined, but the New Zealand Transport Agency was working to a December deadline for its recommendation.
''A new harbour crossing is likely to be needed between 2025 and 2030,'' Key said.
Key said the Auckland Harbour Bridge was one of the most critical transport links in the country, but growth forecasts showed that demand would soon exceed capacity.
''Despite recent strengthening, limits on the weight loading capacity of the clip-ons means heavy truck access may need to be increasingly managed from around 2021,'' he said.
''Congestion on the bridge is already a problem in the peak periods. Traffic forecasts indicate that, as the Auckland economy grows, this will increasingly spread throughout the working day.''
The Government and council have agreed a tunnel was the preferred option.
Also announced today was the combined Auckland Manukau Eastern Transport Initiative and East-West link.
''The area between Onehunga, Mt Wellington and East Tamaki is home to a number of industrial and logistics business that make a critical contribution to the Auckland and national economy,'' Key said.
''Many people are employed in the area and there's considerable growth potential, but the transport links in and out are not up to the job.''
Transport Minister Gerry Brownlee would take advice on how to accelerate the projects.
"It's certainly quite a week for spending money," Key said, referring to the Christchurch rebuild announcements as well as the Auckland transport projects.
The road projects would see improvements to the southern motorway between Manukau and Papakura, an upgrade of State Highway 20A linking Mangare South to the South Western Motorway, and completing a motorway to motorway link between Upper Harbour Highway and the Northern Motorway at Constellation Drive.
Key said the project simply had to be built and if a plan was not set out, growth would be stifled.
''This is all about future proofing both Auckland and New Zealand as best we can.
"If we don't build it we'll be behind the curve and long term we'll pay for that with lower levels of productivity, lower levels of growth, amd higher levels of inflation.''
''It'll be a combination of funding that'll come from proceeds from the asset sales programme money, that will come from national land transport funding; money that we'll simply write out an additional cheque for,'' he said.
''We'll look to do partnerships with the private sector and have some PPPs and obviously the council will have to work through their share.''
The share of the council would be ''significantly lower than ours'' Key said.
Labour's Auckland issues spokesperson Phil Twyford said Key had ''bowed to the inevitable'', claiming the Government had effectively re-announced Brown's existing plan for Auckland.
"National has been in denial for years about the need to get serious about an integrated plan for the future of Auckland's transport system. The need for the City Rail Link, the East West Link and ultimately a second harbour crossing has long been obvious to Aucklanders,'' Twyford said.
"Coming on the back of National's U-turn on the City Rail Link, this week has been a huge win for the popular Labour Mayor Len Brown. He deserves big credit for finally dragging National over the line.''
Twyford said questions remained about how the projects would be funded.
''Where on Earth is the money coming from?''
Labour maintained that the rail link should be fast tracked, which would have it in service ''by the time John Key plans to get his shovel out in 2020''.
Will you be voting in this year's General Election?Related story: Map: Voter enrolment rates declining