Auckland ratepayers are stepping in to plug a $100 million gap in funding from the New Zealand Transport Agency.
The NZTA will replace the funding two years later, in an unusual move allowing new jointly-funded projects in Auckland to begin on time.
The agency is facing a short-term squeeze after the Government's decision not to go ahead with a 1.5 cent per litre rise in fuel excise tax, as well as a fall in other revenue.
It also faces higher costs from the Canterbury earthquakes, and from finishing Auckland motorway projects ahead of schedule.
NZTA regional director Stephen Town says Auckland's amalgamation of local bodies has made the deal possible.
"We don't have to broker an arrangement with seven councils, we've had to just do it with one partner and Auckland Transport so it's been a more straightforward opportunity than if we had been working under the old structure."
Auckland Council transport committee chairman Mike Lee says the city has helped out the Government, yet it has been unhelpful over the proposed downtown rail tunnel.