Auckland City intends dipping into public transport budgets to pay for a trial allowing private cars or even logging trucks into a waterfront bus lane.

Its transport committee decided yesterday to use its least populated bus lane, along 1.8km of Tamaki Drive, for a trial during which any vehicle with at least two occupants will be able to share the lane.

A 4-3 vote followed fierce political debate, and advice from officers that only eight buses an hour ran along the city-bound lane, from Kelly Tarlton's Underwater World to Ngapipi Rd.

Staff said in a report that the lane was uninterrupted by side streets, making it suitable for a trial to gauge any effect on bus travel times.

Passenger transport manager Stuart Knarston said thge numvber of vehicles carrying more than one person was fairly low in Auckland, so the trial was unlikely to cause much inconvenience to bus passengers.

But a decision to pay for the $355,000 trial from unallocated parts of council public transport and walking and cycling budgets drew strong reactions from minority City Vision councillors already upset at what they deemed an attack on the city's 32km network of bus lanes.

"It adds insult to injury," said Graeme Easte.

"Since this is basically a roading project for the benefit of non-public transport vehicles, why is the money not coming out of roading [budgets]? We shouldn't be signalling open season on bus priority lanes."

Citizens and Ratepayers councillor Aaron Bhatnagar said the council needed to do everything possible to move people to public transport, but it should optimise road space, and the trial might promote car-pooling.

The Tamaki Drive bus lane, and one introduced last year in Remuera Rd, had upset many eastern suburbs residents and if the trial produced satisfactory results, "we'll have an opportunity to make people's lives better".

But the staff report said the Tamaki lane established in 2007 had "exceeded expectations" in not only saving buses an average of six minutes a journey, but also in reducing travel times for general traffic by four minutes by improving flows.

Although 20 buses an hour carrying 45 per cent of commuters along Remuera Rd were taking an average of five minutes less than when they had to share their space, the report said, a high level of road-works on the route prevented an accurate determination of the effect on car trips.

Committee deputy chairman John Lister said cars were being delayed by 20 minutes, and council had to listen to community concerns "to get it right".

But independent Gulf Islands councillor Denise Roche said: "If the good people of the eastern suburbs have a choice to catch a fast bus or take a slow car, I believe that's seriously a good thing. If it takes 20 minutes longer, the good thing to do is get in a bus."

Transport general manager Don Munro acknowledged to councillor Cathy Casey that even logging trucks would be allowed into the Tamaki Drive lane as long as they had two or more occupants.

But the trial would include monitoring the effect of the change on other road-users including cyclists, who are allowed in bus lanes.