Public transport growth will see Auckland Council discuss up-sizing its City Rail Link plans to take longer trains

That will see the cost balloon by hundreds of millions of dollars - and means station platforms will have to be longer and the tunnels need to be widened to cater for the longer trains.

The original plans were based on predictions the trains would be carrying 36-thousand passengers per hour by 2045.

But updated information suggestions the council now needs to plan for 54-thousand passengers per hour.

Mayor Phil Goff says doing the extra work now, will save money in the longer term.

The City Rail Link is not the only major public transport to outstrip planners' expectations.

So has the Northern Busway and last year Auckland Transport's HOP card system was used more than 20 million times - that wasn't expected until 2021.