Pointing to a recent case in Australia, Key said light rail projects were notorious for cost blowouts. Labour were also asking the council to pay half of the cost despite its poor financial state, he said.
"Now, mysteriously they'd have $700m. If they really did have $700m then they'd had it for things that are theoretically a higher priority."
Key conceded that National was also guilty of "pork barrel" politics during the recent Northland by-election, when it promised to build upgrade 10 one-way bridges. But he said that was on "a slightly different scale" because it would have cost $60m rather than $1.4b.
Responding to a recent Automobile Association (AA) report which showed congestion had worsened in the last five years, Key said many large roading programmes had slowed traffic. These included work on roads to the airport, the Waterview Connection, the East-West Link, and the Western Ring Route.
Little said yesterday that National had effectively committed to light rail itself through the Auckland Transport Alignment Project and Labour's proposal was simply to bring that forward from 2028-38.
The council has no money for light rail, but new mayor and former Mt Roskill MP Phil Goff has promised to bring forward light rail subject to a business case in the next long-term budget in 2018.
The 13km line would run via Britomart, along Queen St to Dominion Rd, ending near the Stoddard Rd-Sandringham Rd intersection, said Little, who was joined by Labour's candidate in the byelection, Michael Wood, at Dominion Rd in Balmoral.
Little said it was clearly needed to tackle congestion problems.
"Right now, gridlock is choking Auckland's ability to grow. Auckland is crying out for infrastructure projects to get the city moving, but the Government is out of touch and ignoring the problem. Labour will deliver, starting with a modern light rail line," Little said.
Little said the project would cut travel times, reduce congestion and pollution, improve health, and boost productivity. He said the homes of 29,000 people and 48,000 jobs are located within 500m of the planned stops, with major growth likely in the coming decade.