Close-up view of city's newest motorway

Cyclists will be first to try the 4.5km motorway between Hillsborough and New Windsor. Photo / Paul Estcourt

Thousands of people are expected to to inspect Auckland's newest motorway during an open day on Saturday.

But they may have to bring their raincoats.

The Transport Agency is giving a preview of its $201 million Mt Roskill motorway extension before opening it to traffic in stages from Friday next week.

Cyclists will be given first access to the 4.5km motorway between Hillsborough and New Windsor. They will be able to ride up and down it for an hour from 10am, before it is opened to pedestrians from 11am until 3pm.

The agency is providing four access points to the motorway, from Maioro St or Sandringham Rd at its western end, or from its Dominion Rd or Hillsborough Rd interchanges to the east.

Rotary clubs will collect "optional" gold coin donations towards a new mobility lift for the Laura Fergusson Trust and a university scholarship for a Mt Roskill student to be chosen from a low-income family.

Although Saturday's weather outlook is for rain, the event is expected to be popular with residents who have waited for the motorway to be built for decades, and the agency is advising them to avoid traffic congestion by walking or catching buses there.

"Everyone in the area who has waited so long for this - whether they have been for or against it - can't wait to try it out to see if it's a shorter route from home," said local resident and Auckland City Council member Graeme Mulholland.

"I am interested in what impact it will make on the amount of traffic on Richardson Rd and particularly Hillsborough Rd."

Mr Mulholland, who lives on congested Hillsborough Rd, said he had been involved in discussions on the motorway since about 1989.

That was when Mt Roskill Grammar, of which he was a board member at the time, had to obtain permission to build a childhood centre on land designated for a motorway and a railway line.

The designation was moved to the other side of Somerset Rd, into an edge of Keith Hay Park, as part of an exercise to straighten the route.

But that took it through part of the Akarana golf course next to Dominion Rd, meaning some of the course's 18 holes had to be redesigned.

After concern over the motorway's effect on the Mt Roskill volcanic cone, the Dominion Rd interchange was also redesigned, producing what Mr Mulholland said was a superior result.

"I think they have made a marvellous job of landscaping, softening the cut of the motorway," he said.

Transport Agency regional director Wayne McDonald said it was part of the original motorway masterplan for Auckland dating back to 1965.

But well before that, the route was gazetted for a railway linking Southdown to Avondale, for which the motorway builders have made future provision beneath the four traffic bridges and two walking and cycling paths crossing their road.