UPDATED 12.22PM Construction on Auckland's $2.5 billion City Rail Link project is now officially under way, after a ground-breaking ceremony at Britomart this morning.

SEE ALSO: Construction of Auckland's Rail Link begins today

Auckland Mayor Len Brown said it was a historic day for the city.

"Today changes Auckland for ever. The CRL is the heart of dealing with the city's growth, with propelling our economy, and creating a future Aucklanders want."

Mr Brown said the rail link would provide the "single best solution" to meet the demand for access to the city by allowing 30,000 people an hour to move during peak travel times.

Ready to make history #CRL #KeepingAucklandMoving https://t.co/DePjYeBwUb

— Len Brown (@mayorlenbrown) June 1, 2016

Auckland Transport is forecasting an 88 per cent increase in rail patronage into the city the CRL's first year of operation and a 40 per cent increase across the network in the morning peak.

Mr Brown said more than 70 private projects worth $7 billion are expected to be built around the CRL and completed by the time the link opens in 2022/23.

"The CRL has been my number one priority since becoming the first Mayor of the united Auckland. Today I am incredibly proud to stand with the Prime Minister and Minister of Transport to ensure this project proceeds for the future benefit of all Aucklanders," the mayor said.

Prime Minister John Key told the ceremony this morning that the way Aucklanders travel is changing.

"They're not going to own cars - they're going to get on the City Rail Link, they're going to get on the train, for transportation they'll get on the bus."

Great to see the #CRL is under way! #finally https://t.co/WRI0Ql6Nhk

— Phil Goff (@phil_goff) June 1, 2016

Campaigners are using the start of the project to urge officials to extend the rail network to the North Shore.

Campaign for Better Transport spokesman Cameron Pitches said the time was right to start planning to extend the rail network to the North Shore.

"It seems obvious that a high capacity, low carbon emission electric rail link across the Waitemata will do far more to move the thousands of people necessary at peak times each working day."

Mr Pitches said a road-only tunnel between Victoria Park and Esmonde Rd was estimated to cost between $4b and $6b and smaller diameter tunnels for rail - the same size as the CRL tunnels - have been estimated to cost up to $1.5b.

Green Party transport spokesperson Julie Anne Genter said Auckland's population growth meant the Government needed to urgently prioritise extending the rail and bus networks.

"Now that the CRL is under construction, the Government must start planning for rail to the airport and across the harbour, to help the growing number of people living on the North Shore get into the city.

"The CRL is just the first step in the development of a high quality commuter rail and public transport system in Auckland, but it's a big step because it shows what's possible," said Ms Genter.

Finally! #CRLgroundbreaking 💚🚇❤️ with @jacindaardern pic.twitter.com/dYcIT8CRH3

— Julie Anne Genter (@JulieAnneGenter) June 1, 2016

Transport lobby group Generation Zero said the long planned project was finally underway thanks to the tireless effort of Auckland Transport and Auckland Council who fought for the CRL for years.

"The CRL is the heart of a much needed rapid transport network in Auckland. But it can't stop here. We need rail to the North Shore, rail to the airport, light rail in the isthmus, there is plenty left to do."