National are defending the cost of their newly announced $24 billion transport policy.

Labour's already poked holes in it, saying the four-lane highways between Whangarei and Tauranga plan alone is out by billions.

But Opposition transport spokesperson Simeon Brown says this funding covers the first four stages - of what will be a multi-generational project.

"This is just like the Waikato Expressway when that was started, it was done in a number of stages over many years."

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