Prime Minister Chris Hipkins says National is "dreaming" with the party's estimated costs to build fresh roads across New Zealand.
The Labour leader said National was either willfully negligent or not being truthful to the public.
"There's more publicly available information that shows cost estimates that they are using are massively out," he told AM on Tuesday.
"They are promising a four-lane highway from Whangārei to Tauranga and they're saying that's going to cost $6 billion - that's just la-la land dreaming. There's just no way you could build a four-lane highway across that."
It comes after National unveiled its transport policy on Monday, which Labour claimed was billions of dollars short.
"One of the costings that they released was a costing that was produced in the time that they were last in Government," Hipkins said.
"[Since then], just in the last two years alone, the price of bitumen has gone up by over 100 percent. So the idea that you can just pluck these historic figures out and say, 'Oh, that's what we're going to build roads for' is just dreaming."
National campaign chair Chris Bishop has already had a lot to say in response to Labour's claims.
"Let's not forget that the very costings that National provided - and added a healthy contingency to - were the figures supplied by [Transport Minister] David Parker's office - and the only figures publicly available," he said.
National's fellow right-bloc party ACT also likened National's plan to a "wish list".
ACT transport spokesperson Simon Court said New Zealanders would be "lucky to ever drive on the completed roads" announced by National under current funding models.
Hipkins said Labour believed the costings for four roads announced by National were out by several billion dollars.
"The National Party has chosen to use out-of-date costing information," he said. "Whatever claims they're making, there is publicly available information that shows that the costings they're putting out there are just plain wrong."
Bishop on Tuesday called Labour "in chaos" over transport.
"While National rolls out the most comprehensive transport plan in decades to get New Zealanders where they need to go faster and safer and drive the rebuild of our economy, Labour is once again in a state of chaos, this time over its transport plans," he said.
