Bids for stage one of the country's most expensive infrastructure project are down to a shortlist of three groups, made of local and international companies

The Northland Express contract-winner will design and build four lanes spanning 26-kilometres from Warkworth to Te Hana - including an 850-metre-long tunnel.

Herald political editor Adam Pearse says the infrastructure commission's estimated it to be the biggest ever investment of its kind.

A group's expected to be chosen early next year, witrh construction to follow by year's end.