Labour's truck ban could cover 0.7 per cent or 7 per cent of New Zealand's motorways - depending on who you listen to.
There was some confusion about the impact of the policy, which was unveiled by leader David Cunliffe on Tuesday.
It would block trucks from using the outsidelane on three or four-lane highways in an attempt to reduce congestion, especially logjams during public holidays.
Economic Development Minister Steven Joyce mocked the policy in the House yesterday, saying it would apply on just 50km or 60km of New Zealand's 11,000km of state highway.
This contrasted with the New Zealand Transport Agency's analysis. The agency said that there were 840km of highways with three or more lanes - 160km in Auckland.
Mr Joyce, a former Transport Minister, questioned this figure, saying that the only wide motorways were in parts of Auckland, outside Wellington, and near Wainuiomata.
"I'd like there to be more, and we're building some, but we're a long way from it having a significant impact in any way, shape or form on the highway network in New Zealand."
NZTA later clarified that the 840km referred to state highways with a total width of three or more lanes, not three or four lanes in each direction.
The proportion of motorway which had three or more lanes each way was 68km - or around 0.7 per cent of the total network.
A large proportion of the wider motorways were in the Auckland region, where Labour said the policy would be most popular.
Deputy leader David Parker said: "You ask the 1.5 million people in Auckland and they will say it annoys them that at times the fast lane is blocked by trucks that are only meant to be doing 90km/h."
Labour also revealed that it would scrap registrations for caravans and light trailers and reduce road user charges for caravans and motorhomes.
