Driving on Auckland's motorways could cost up to $6 each weekday morning in a tolling option which is among five anti-congestion charging schemes under Government consideration.

Potential charges outlined in a $2.3 million Ministry of Transport study include $4.80 for driving up the Southern Motorway from Drury to Auckland, $4 along the Northwestern Motorway from Massey, and $3.35 from Greville Rd to the city side of the harbour bridge.

But motorists using a combination of these links between 6am and 10am, such as those travelling between Northland and the Waikato, will not have to pay more than $6 a day.

The scheme is the only option for which charges depend on distance travelled - motorists will pay either 25c or 15c a kilometre, depending on which parts of the network are in the most dire need of unclogging.

Transit NZ is also separately considering tolling parts or all of the region's yet-to-be completed western ring route of motorways all day long, to bridge an $860 million construction funding gap for projects including the $1 billion Mt Albert to Waterview link.

But it has yet to announce possible toll prices, and has deferred public consultation indefinitely while the ministry study is being worked on.

Not all stretches of motorway would be tolled under the ministry's option - which is in response to computer modelling of 20 per cent more congestion in Auckland by 2016 - but some limited-access arterial routes could be added to the mix.

Westbound traffic would get a free run on the Northwestern Motorway, for example, as would southbound vehicles once past Mt Wellington on the Southern Motorway.

Commuters would pay $2.50 to travel 10km from Spaghetti Junction to Mt Wellington at 25c a km, but their northbound counterparts could drive more than three times that distance from Drury to central Auckland for $4.80c at 15c a km.

It could cost eastbound commuters $4 to travel the 16km length of the Northwestern Motorway, but those driving south from Greville Rd on the Northern Motorway could make the 17km trip into Auckland for $3.35c.

That is because the 15c charge would apply to the first section of their trip, and rise to 25c as traffic clogs up closer to the harbour bridge.

Although that may sound complicated enough to the average motorist just trying to get to work to earn money to pay the charge, the ministry refrained from considering even more complex systems used overseas.

In places such as Singapore and California, toll charges are adjusted throughout each day to reduce or increase traffic volumes to maintain desirable average speeds.

A ministry spokesman ruled that out yesterday, saying charges would remain fixed from day to day if a distance-based scheme was adopted in Auckland, to keep things "simple."

But the scheme's adoption is looking unlikely, given serious drawbacks identified by the ministry study.

Large volumes of traffic would be diverted elsewhere, as drivers search for more circuitous but free routes to work, meaning considerable sums would have to be spent improving local roads to cope with extra congestion on these. Operating costs would gobble up $65 million of gross revenues of around $86 million in the first year

And after paying for local roading improvements and extra public transport services, a requirement of any such scheme, the strategic network option is picked to lose $257 million in today's dollars over 20 years.

This compares with positive net values of up to $460 million for other options, and sets it squarely against a Cabinet directive that any scheme must raise enough money to cover its cost and those of mitigation measures.

The feeble financial predictions for the scheme are believed to be casting a shadow over Transit's own hopes for tolling the western ring route, although that agency says a day-and-night charge would raise much more money to repay road-building loans.