Sports clubs, schools and churches are among scores of organisations being blitzed by an email campaign warning of a closure of Newmarket Viaduct's southbound motorway lanes early next month.

"No, this is not a hoax," says a message on the Transport Agency's website, in reaction to inquiries from some disbelieving email recipients about the closure of one of New Zealand's busiest motorway arteries for up to 36 hours from 5pm on Saturday, September 4.

The agency hopes to take less time than that to switch traffic lanes to a new southbound structure built as the first stage in its $215 million viaduct replacement project.

But it wants some leeway in case of any hitches in moving an 800-tonne piece of equipment on to the existing southbound carriageway, which will be demolished to make way for a new northbound structure.

That machine is the giant blue lifting gantry that has dominated the Newmarket skyline since the beginning of this year, and which will be moved by its own power generator on large metal sliding beams, from on top of the new southbound structure it has just finished building.

Drivers will be urged to stay as far away from Newmarket as possible, to the extent of using parts of the western ring route between Albany and Manukau to avoid traffic jams.

Traffic engineers are also considering closing some city entry points to the Southern Motorway - such as those from Hobson St, Symonds St and Khyber Pass Rd - to ease pressure on the Gillies Ave off-ramp at the north end of the viaduct.

The Transport Agency is negotiating with Auckland City to create a two-lane clearway through Newmarket streets from the off-ramp via Mortimer Pass for drivers who ignore electronic message boards warning them to leave the motorway earlier.

That has unsettled Newmarket Business Association chief executive Cameron Brewer, who said there would be too much pressure on Mortimer Pass even if the agency persuaded 60,000 of the 80,000 southbound drivers who would normally cross the viaduct on a Sunday to leave the motorway before Gillies Ave.

He predicted mayhem, saying: "I can't understand why they need to push it all through a Newmarket side street".

Mr Brewer said he would prefer vehicles to be directed off the motorway through Grafton Gully, or for one of the viaduct's northbound lanes to be used for a limited flow of southbound traffic.

Agency highways manager Tommy Parker said that "contraflow" idea had been considered but rejected for safety and other reasons, but Mr Brewer's concerns would be taken into account as plans for the closure were refined.

Although the closure will coincide with Father's Day, Mr Parker said there was usually less traffic on that date than on other Sundays.

Plans by the Auckland Regional Transport Authority to run extra trains during the closure would not have been possible on the weekend before, as railway lines will be closed for construction and maintenance.

At least four important sports events are being held over the closure weekend, including a netball test with Australia at Vector Arena, provincial rugby matches at Eden Park and North Harbour Stadium and a two-day badminton tournament off Gillies Ave.

Organisers have been asked to spread the message about a need for car-pooling, planning alternative routes or using public transport.

Although the Transport Agency's construction consortium is using emails and Facebook to reach as many drivers as possible, it is also chewing through a budget of about $100,000 for more traditional advertising which has started in newspapers and will be on radio stations from next week.