One of the biggest risks in building the $3.4 billion City Rail Link will come towards the end when operating systems are installed - something that potentially could cause lengthy delays and extra costs.

The project's chief executive Dr Sean Sweeney says the real challenge of New Zealand's largest infrastructure project is not in building the tunnels or stations but in the systems for controls, signals and safety which are added at the end.

He cites system problems with London's $30b Crossrail project that delayed works for three years and added an extra $5b as an indication of where the

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