Independent air-quality specialists have criticised a Transport Agency assessment of its $1.75 billion Waterview motorways project for using average vehicle volumes to predict pollution levels.

They say the agency should instead have modelled the impact of vehicle emissions on a heavy traffic day, when hourly emissions of harmful tiny particulates could be four times higher than its predictions for surface road sections of the project.

The Emission Impossible consultancy also says in a report prepared for a consents hearing next month that it is "extremely concerned" about an assumption that a 4.5km extension of the Southwestern Motorway to be joined to a widened Northwestern Motorway will not create much extra traffic demand.

But in their evaluation, commissioned by the Government's Environmental Protection Authority, consultants Jayne Metcalfe and Rachael Nicholl agree with the Transport Agency that filtering vehicle fumes from a 2.5km length of twin motorway tunnels is unlikely to be cost-effective.

That follows another report commissioned by the authority, from the Environmental Management Services consultancy, which suggests the agency consider filtering fumes as a way of reducing the height of proposed 25m to 27m venting towers, to make them more visually acceptable to neighbouring communities.

Although the Emission Impossible consultants "generally agree" with an agency expert that ground-level concentrations of untreated contaminants from the tunnels will be well within acceptable levels, they are in accord with Environmental Management Services that alternative tower heights have not been adequately considered.

They say limited information provided by the agency suggests a 15m tower may result in similar effects to a 25m stack.

And despite their satisfaction that localised contamination from the two towers will not be significant, they indicate strong concern about the potential pollution impact from surface sections of the new and enlarged motorways.

The consultants are concerned about potential impacts of more traffic on homes close to the Northwestern Motorway and near Alan Wood Reserve and Hendon Park in Owairaka, which the surface part of the new link will subdivide.

They note the most harmful type of vehicle pollution exceeded a regional air quality target in Alan Wood Reserve four times between June and August last year.

Although much of the pollution was blamed on domestic fires, they say that effectively means "the airshed is over-allocated ..."

The consultants are particularly scathing of an estimate in Transport Agency documentation that "induced" traffic - meaning vehicles which would not otherwise be driven without access to new roads - will amount to an increase in vehicle trips of just 0.06 per cent, or 2400 more trips a day.

"This does not seem realistic," they say, noting a reference in an Auckland Regional Public Health Service submission to United States research estimating that nearly three quarters of every percentage increase in roading capacity is absorbed by induced traffic.