Auditors of Auckland's $1.75 billion Waterview motorways project want the Transport Agency to consider filtering vehicle fumes instead of pumping them untreated from tunnel venting towers.

The agency says treating the fumes would be costly and unnecessary, as they would be dispersed from 25m to 27m towers in tunnels between Owairaka and an interchange with the Northwestern Motorway.

The tunnels on a new 4.5km motorway link would improve air quality by cutting surface road traffic.

The project includes widening the Northwestern Motorway between St Lukes and Te Atatu.

But Environmental Management Services (EMS) says in a report for a consents hearing next month that community perceptions of adverse effects could be addressed by a commitment to treat the tunnel emissions.

The report, commissioned by the Environmental Protection Authority for a Government-appointed Board of Inquiry, notes tunnel equipment has been designed to accommodate treatment systems if needed in the future.

"This begs the question in our minds, if treatment was applied now, would this enable a lower stack of, say, 15 metres with consequent reductions in landscape and visual impacts for the local community and particularly the school," the consultants say.

They refer to Waterview Primary School and the kindergarten, outside which the Transport Agency wants to build the 25m northern tower and ventilation building in Great North Rd.

A 27m tower and associated building is proposed for the Owairaka end of the tunnels, in Alan Wood Park, which a landscape expert acknowledged would "supplant" most of an area of open space between Oakley Creek and houses in Hendon Ave.

Landscape architect Stephen Brown, in a report on the consents applications, said the effects on the park would be profound.

"Its very industrial and rather utilitarian form will effectively curtail the residual open space extending south from New North Rd and impose itself on neighbouring properties in a most unfortunate and intrusive manner."

EMS consultants say Mr Brown's initial assessment was "about as damning as could be conceivable in the context of this open space and residential development".

In response to public submissions, the tunnel designs had been modified.

Mr Brown says in a brief of evidence for the consents hearing that while the structures at both tunnel portals remain of concern the overall project at Waterview and Owairaka was acceptable. The revised designs were "very positive and entirely compatible with the local landscape".

EMS says his conclusion is surprising compared with his other comments, and it recommends the agency inquire further into technical and cost constraints of putting much of the southern building underground.

Air quality consultant Gavin Fisher in his evidence statement says it would be almost impossible to remove all contaminants from vehicle fumes and "hugely expensive" to install and operate filters "for almost no benefit to the community or the environment."

Emissions would not be particularly visible, he says.

"They will be nothing like the older-style perception of a billowing brown or black smoke plume. Once the vent buildings become an accepted part of the visual landscape of the area, residents will hardly be aware they are discharging anything."