Sir Brian Roche has resigned from the board of City Rail Link (CRL), having been chair of the board since CRL Ltd was established in 2017.
He will continue to serve on the board until a replacement is found - likely around December.
CRL is tasked with delivering Auckland's new underground rail loop and new stations. It is one of the biggest infrastructure projects attempted in New Zealand in recent years.
Roche said that he was stepping away from the job now so that someone else could step in and see it through until CRL opens in 2024.
"The project has another few years to run and this is the time to get someone who can see it through to the end," Roche said.