UPDATED 5:30PM: Prime Minister John Key has talked to Larry Williams about the government's decision to fund the Auckland City Rail Link.

Government funding for Auckland's ambitious $2.5b City Rail Link will be brought forward to 2018, two years earlier than planned. 

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Prime Minister John Key revealed the decision this afternoon at his State of the Nation speech to the Auckland Chamber of Commerce. 

Key had already signalled the speech will reveal big new infrastructure spending, stating that the country needs Auckland to succeed as it's the nation's biggest commercial centre and main gateway for international tourists.

The Prime Minister also announced an East-West connection between the SH1 and SH20 motorways will be upgraded to a project of national significance. 

Political editor Barry Soper reports that local political considerations have been at play during the announcement.

"Len Brown is here in all his splendour, and he will no doubt be taking some credit for it."

"But John Key was met at the door of the Langham not by Len Brown but by Michael Barnett, who of course could be a contender for Mayoralty."

Aucklanders have been waiting for this transformation for decades #CRL #KeepingAucklandMoving https://t.co/WqZ1Oq4IIL

— Len Brown (@mayorlenbrown) January 27, 2016

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Prior to the speech Chamber of Commerce head Michael Barnett said that fast-tracking the CRL funding was vital.

"For forty years we've done nothing," Barnett said. "We're in catch-up mode. So whether it's housing or whether it's infrastructure, what we need now are decisions and a speed of application for some of these projects."

The Green Party earlier claimed funding the CRL would be a victory as co-leader Metiria Turei believes the CRL is another Greens idea that the government has decided to support.

"We've campaigned for the central rail link for a very long time now and government is finally coming to the party on this," she said.

What is the City Rail Link?

A 3.4km rail tunnel up to 42m below ground from Britomart to Mt Eden will have two new stations, Aotea and Karangahape Rd. The link is costed at $2.5 billion and, according to Mayor Len Brown, will have wider economic and environmental benefits beyond the central city. Britomart, which can handle 20 trains an hour will become a through station handling 48 trains an hour. It'll save some west Auckland commuters an average of 17 minutes per trip.