The Finance and Transport ministers have signed an agreement with Auckland mayor Phil Goff to create a new entity in charge of finishing the City Rail Link.
City Rail Link Limited (CRLL), a company jointly owned by central and local government, is now responsible for the project, which is estimated to cost between $2.8 billion and $3.4 billion.
Retired NZ Post boss Sir Brian Roche will chair the company.
Transport Minister Simon Bridges said the CRL was Auckland's top transport priority.
"It will double the capacity of the whole existing rail network and provide significant travel time savings for commuters.
"Auckland's population predicted to grow by more than 700,000 people over the next 30 years, the CRL will play an important role in getting people in and out of the city with ease."
Building the country's first underground rail system was a "massive" project, he said, and the deal was a milestone.
Finance Minister Steven Joyce said the partnership was "crucial" and would ensure a high-quality result.
"This is a complex and critical piece of infrastructure that will unlock major development opportunities across central Auckland."
Once complete, the 3.4km double-track underground rail line will run from Britomart station through the CBD, connecting with the existing western line at Mt Eden station.