Aucklanders are being warned of possible delays to bus services throughout the region during the country's biggest motorway closure, which is just eight days away.

The Transport Agency has also confirmed that as well as closing Newmarket Viaduct's southbound carriageway for up to 36 hours from 5pm on Saturday, September 4, it will block three city entry points to the Southern Motorway - at Hobson St, Symonds St and Khyber Pass Rd.

Although its aim is to reduce the load on the Gillies Ave off-ramp just short of the viaduct, that will extend the geographic range of local roads likely to be disrupted by the closure.

Even so, Auckland City transport committee chairman Ken Baguley said yesterday that he was confident traffic engineers from both the agency and the council were doing all they could to minimise the impact of the disruption, which was the price of completing "an incredible bit of engineering".

He was referring to a $215 million project to replace the Newmarket Viaduct.

A new southbound carriageway is to become available to traffic on the Monday after the closure, which is needed to switch lanes between structures and move an 800-tonne lifting gantry.

Mr Baguley said teams from both organisations would be busy throughout the closure monitoring traffic movements and adjusting signals if necessary to reduce logjams.

Transport Agency highways manager Tommy Parker said the risk in allowing cars on to the motorway from the central city would have been seeing them heading "straight into a queue they can't get out of".

The Auckland Regional Transport Authority is promoting trains as the fastest and most reliable way to travel during the closure.

It is laying on more trains than usual, and with extra carriages, in a desperate attempt to keep Auckland moving.

But authority spokeswoman Sharon Hunter said widespread road congestion as a result of diverting up to 60,000 cars, which would normally use the viaduct's southbound lanes on a Sunday, would also hold up buses, with potential region-wide effects.

The authority has arranged for Veolia Transport to add six extra train services on the Saturday of the closure, and 25 more on the Sunday, between Otahuhu and Britomart.

It will also subsidise three extra ferry services each way between Auckland and Half Moon Bay on Saturday, and five on Sunday.

Although a contributor to a Facebook page set up by the Transport Agency's viaduct-building consortium suggested public transport be provided free during the closure, Ms Hunter said that would be a costly proposition for which her organisation had received no request from the agency.

Mr Parker said it was unclear whether free public transport would make any substantial difference to traffic congestion.

"What we are doing is asking people not to travel," he said.

The Facebook page includes a list of local events to encourage Aucklanders to stay close to home.

Although closing the Hobson St, Symonds St and Khyber Pass Rd entry points to the motorway is intended to reduce pressure on the Gillies Ave off-ramp, the agency has upset the Newmarket Business Association by preparing a two-lane clearway through its territory to the St Marks Rd on-ramp via Mortimer Pass.

It has widened the Gillies Ave and Mortimer Pass intersection temporarily to cope with up to 2000 vehicles an hour including side-to-side trucks, compared with normal peak southbound flow across the viaduct of 4300.

But association chief executive Cameron Brewer said: "We just think it's ludicrous to divert motorway traffic through one of the smallest streets in Auckland."

Mr Brewer said his organisation strongly supported the viaduct replacement project and was looking forward to a public walk across the new carriageway between 10am and 2pm on Sunday, to be followed by a cycle ride from 2.45pm.

"It's a great opportunity to showcase Newmarket and for families to come in and be part of history and support local cafes and shops because the next weekend [of the viaduct closure] will be a whole different proposition," he said.

Mr Parker said that although a second closure would be needed in 2012 to switch traffic to a new northbound carriageway, there had been little disruption to traffic during the rest of the project.

"We think two days of disruption is reasonable in the scale of things, but we will hopefully get off [the viaduct] as quickly as we can."

GEARING UP
* Southbound lanes of Southern Motorway will be closed from 5pm on Saturday, September 4, and all day Sunday, September 5.
* Hobson St, Symonds St and Khyber Pass Rd on-ramps will also be closed.
* Bus services across the city may experience delays.
* Motorists are advised to travel by car only if necessary over this period, especially on the Sunday.

For detours, maps and more information go to: www.nzta.govt.nz/newmarketconnection, www.maxx.co.nz