The State Highway 20 extension faces further delays as a group challenges Transit New Zealand in the Court of Appeal to save the Mt Roskill volcanic cone.
Construction of the 4km Hillsborough Rd to Mt Roskill link was due to begin early next year.
Two weeks ago theVolcanic Cones Society lost its Environment Court challenge to stop the road cutting through the volcanic cone, although the court accepted it had a unique history.
"It is therefore important in terms of the RMA [Resource Management Act] that Mt Roskill, one of the relatively unspoilt examples of this past history and activity, be protected as required by the RMA from inappropriate use and development," the court decision said.
The society wanted to stop the Dominion Rd interchange from cutting into the cone by moving it 400m to May Rd. It is challenging the court's decision that the effects on traffic and a future railway station are more important than the cone.
Society committee member Greg Smith said the group was not trying to hold up the motorway.
"We want to protect the cone and Transit wants to build the highway. We feel the two things can be done."
The society had spent $30,000 on court costs so far, including hiring a traffic engineer to present different options. Transit rejected a tunnel because of the soil and a motorway bridge was too expensive.
The court recognised the society's right to protect the volcanic cone and said that Transit and the local councils agreed that the motorway should be moved away if at all possible.
The court did not rule for costs against the society but did for another group, Greenbelt, which had challenged the project on the grounds that it would ruin wetlands, local creeks, the cone and the health of nearby residents.
Transit is expected to ask for a priority hearing so it can begin construction in the new year.
