But the agency believes that number is likely to have been reduced by high noise walls erected when the motorway was being widened.
Regional highways manager Tommy Parker acknowledged yesterday that lining up funds had taken longer than hoped, with complex negotiations over cost-sharing compounded by Auckland's local governance changes.
His organisation had worked closely with community leaders for eight years to secure funding and approval for the bridge, for which it will pay 68 per cent of the cost, leaving the other 32 per cent to be covered by Auckland Transport.
Another agency spokesman said the cost increase from $2.5 million followed a redesign of an earlier proposal, to make the bridge 3m wide rather than 2m proposed initially.
That is to allow pedestrians and cyclists enough room to pass each other safely and comfortably.
The start of construction was marked by a ceremony at which students of Guile Jackson's old school planted trees in Manutewhau Reserve. The bridge will stretch from there across the motorway to Westgate Drive.
Commuters are meanwhile about to face delays in two other busy parts of Auckland - on the Ellerslie-Panmure Highway and the Northern Busway - to allow major construction work.
Auckland Transport will reduce the speed limit to 30km/h on the highway from Monday until the end of the year from Mt Wellington Highway to the Panmure roundabout to build a new bridge over the eastern railway line.
The bridge, which will carry a busway to Panmure's railway station, is part of a $180 million works schedule including a new road to Glen Innes, the first stage of the $1.5 billion Ameti package of transport improvements though Southeast Auckland.
Northern Busway passengers can also expect delays from the following week for the rest of this year as Transpower threads high-voltage cable through service bays along the route as part of a $415 million project to bolster electricity supplies to Auckland and Northland.
It hopes that by reducing the speed limit for buses from 80km/h to 50km/h between Constellation Drive and Wairau Rd, it can keep delays to between one and four minutes.
BRIDGE FACTS
$6m for the new footbridge towards the upper end of Auckland's Northwestern Motorway
209m long to reach across the motorway
28 people crossed the motorway in the area each weekday, rising to 48 on Saturdays