A cost blowout on Auckland's biggest public transport project has come as no surprise to construction experts.
The City Rail Link (CRL) is now expected to cost more than $4.4 billion - up from a previous estimate of $3.4 billion.
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The upgraded cost was announced on Wednesday after what CRL CEO Dr Sean Sweeney called a "rigorous and comprehensive review" of the project.
Professor John Tookey told Newshub the project's expansion, and city growth, are factors in the new cost.
"What folks tend to forget is that we have a finite capacity to do stuff, there are only so many people with the skills and so many contractors in the city and they have a finite capacity, you can only expand so much."
Prof Tookey said Auckland is undergoing a construction boom across both the residential and commercial sectors.
"We've recently hit a 40 year high in consents associated with housing consents and then you've got major projects going in, the likes of the convention centre and similar."
On top of this, the previous estimate was from 2014, which means a lot of costs needed to be reassessed under today's prices.
"Engineering needs to be recalculated, redeveloped and it just spirals as a result of 'minor changes', when you're dealing with a moving target, you're always going to be guessing in effect."
It was announced last year the CRL would be expanded to fit longer nine car trains at stations, accommodating for future growth.
The project, which is expected to open in 2024, will allow for more rapid transit across the network and introduce new stations at Aotea and Karangahape Rd.