Plans to expand the City Rail Link have been agreed upon by Auckland Council and the Government.

The decision comes after original projections for the Rail Link were found not to reflect rocketing growth in rail patronage across Auckland.

The new agreement means the tendering process can now consider; widening tunnel sizes, lengthening platforms at new rail stations to cater for nine carriage trains - rather than six, a second entrance for the Karangahape Rd station and other associated station work.

Auckland City Rail Link.

A result of new estimates predicted that City Rail stations need to cope with the capacity of 54,000 passengers an hour at peak, rather than the original estimates of 36,000 by 2035.

Auckland Mayor Phil Goff said the growth in rail travel is a success story for Auckland.

"The growth in popularity of rail travel in Auckland required council to take the decision today to increase our investment in the CRL and expand new rail stations to cater for the huge number of people who will be commuting by rail in the next ten years," he said.

"Getting the work done now while CRL is still under construction will avoid retrofitting the system which would double the cost and require the tunnels to be closed for two years for widening within a decade of it being opened."

CRL Aotea Station upper level skylights interior design.

Transport Minister Phil Twyford said these changes would ensure that when CRL opens in 2024, Aucklanders will get a modern and efficient rail service that benefits the entire transport network for decades to come.

"A decade of under investment in transport infrastructure has bought Auckland to a near standstill," Twyford said.

"Today's decision has allowed us to avoid repeating the mistakes of the past when within 10 years of opening the Auckland Harbour Bridge it had to be expanded from four lanes to eight."

CRL Mt Eden station interior design.

The Council's Governing Body voted overwhelmingly to expand work on the City Rail Link.

Costs associated with expanding the scope of work for CRL are confidential while the tender process to procure the work is underway. Exact costs will be known more precisely early next year once tenders are received.

City Rail Link costs are shared equally between Government and Auckland Council.

More to come.