Auckland's newest motorway, through Mt Roskill, has been opened to traffic in both directions after fine weekend weather allowed final road markings to be painted at westbound ramps.

Only eastbound lanes of the 4.5km extension of State Highway 20 were available for traffic after the opening ceremony on Friday afternoon, leaving contractors Fulton Hogan working towards a deadline of this morning to clear the westbound carriageway from Hillsborough to New Windsor.

But a fine Saturday let them complete the markings in time for the $201 million motorway to be fully available to traffic yesterday.

Initial traffic was light but steady, but the real test will be this morning. Hillsborough Rd residents above Manukau Harbour hope it will become the main commuting route between West and South Auckland, removing bumper-to-bumper congestion from their midst.

The latest addition to the Southwestern Motorway has interchanges at Hillsborough and Dominion Rd, but ends on a roundabout shared by extensions of Sandringham Rd and Maioro St.

That has prompted the Transport Agency to use an electronic message board to warn drivers to slow from 100km/h from within about 350m of the roundabout, where a 50km/h limit starts.

Saturday's weather also allowed Auckland City contractors to lay new seal over the original length of Maioro St through New Windsor, which the agency has widened to two lanes in each direction for extra traffic between the motorway and Waitakere City.

The agency hopes the roundabout will be temporary only, to be replaced in 2015 by an interchange marking the start of the controversial 5km Waterview link between the Southwestern and Northwestern motorways.

Transport Minister Steven Joyce said at the opening ceremony that the Mt Roskill extension was another "piece of the jigsaw" of a critical transport corridor needed to take pressure off State Highway 1 and the harbour bridge.

He noted other recent milestones on the western ring route, including the transfer of traffic on the Mangere side of Manukau Harbour to a new southbound motorway carriageway matching a duplicate harbour crossing to be completed for $230 million by 2011.

Another was the erection last week of the final beam of a 240m flyover of the SH1 Motorway at Manukau, to form a northwest connection with the southern end of the ring route, a project to be completed next year for $210 million. Mr Joyce said even a $1.4 billion "reasonable cost" estimate produced by the Transport Agency last week for a combined surface and tunnelled motorway through Waterview would be seven times that of the Mt Roskill extension, although the sum includes $240 million to widen and raise the Northwestern Motorway's nearby harbour causeway.

The total is $809 million less than an estimated construction cost of $2.2 billion for digging two-lane twin tunnels under most of the Waterview route and widening the causeway.

But Avondale Community Board member Paul Davie is sceptical about the claimed cost advantage of the new scheme, given that it involves digging 1.8km of tunnels wide enough to add a third lane in each direction if warranted by future traffic demand.

That compares with 3.2km of twin tunnels under a more direct route.

Auckland University senior economics lecturer Rhema Vaithianathan is questioning an addition by the Treasury of $550 million in financing costs to a business case for the doomed twin tunnels, saying the $809 million construction cost difference does not justify that.

Dr Vaithianathan, a Mt Albert resident and former Treasury employee, said the assumed financing costs appeared arbitrary and "make no sense".

The Government says the cheaper scheme will escape such costs as it will be paid entirely from an enhanced national land transport fund, but Dr Vaithianathan wondered why financing for the twin tunnels could not have been spread between the fund and supplementary borrowings.

RINGING THE REGION

* Southwestern Motorway, Onehunga to Hillsborough - 2km, completed 1989 for $8 million.
* Mt Roskill extension - 4.5km, completed at the weekend for $201 million.
* Manukau Harbour crossing duplication - completion 2011 for $230 million.
* Manukau link between state highways 1 and 20 - 4.5km, completion 2010 for $210 million.
* Hobsonville link between Northwestern Motorway and Greenhithe - 6km, completion 2012 for $220 million.
* Waterview connection plus widening Northwestern causeway - 4.8km, possible completion 2015 for $1.4 billion.