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Preliminary work on Auckland's $1 billion-plus Waterview motorway is to start early next month, surprising its opponents.

The Transport Agency, which is holding a series of community information sessions, has confirmed plans for about $10 million of "enabling" works at the southern end of the proposed 4.5km semi-underground link between the Southwestern and Northwestern Motorways.

These will include diverting a sewer line and a tributary of Oakley Creek, ready for the first half of a motorway interchange with Maioro St in New Windsor to be built for $40 million over 14 months from September.

Tunnel or Nothing group spokeswoman Margi Watson has questioned the early start, as the agency has yet to lodge land designation and resource consent applications for the motorway.

Its construction is expected to start late next year for completion in 2015 or 2016.

But principal project manager Clive Fuhr said the first half of the interchange, with southeast-facing on and off-ramps, was needed for the Mt Roskill motorway extension, which now ends in a roundabout pending construction of the Waterview link.

The interchange will be west of the roundabout, and will include an extension of Maioro St into a bridge to Stoddard Rd.

The existing extension of Maioro St to the roundabout will become a motorway off-ramp.

The Transport Agency says it intends digging about 840,000cu m of material for a 2.5km pair of tunnels, some of which it is considering using for raising a flood-threatened marine causeway along the Northwestern Motorway as part of the $1.4 billion project.

Information sessions will be held from 6pm to 8.30pm on Wednesday at the Waterview Methodist Church and from 10am to 5pm on Saturday at Owairaka Primary School.