Opponents of a $470 million-plus highway through southern Auckland say engineers are ignoring potentially disastrous effects on a premier park.

Papakura resident Robin Harger, a retired United Nations ecologist, has alarmed Auckland Council members with advice that upper gullies of Manurewa's 216ha Totara Park have already suffered "a massive dieback" of bush after a far smaller roading upgrade 15 years ago altered water flows.

He says that killed a large number of mature totara trees in their namesake park, and left gullies lined with dead ponga trunks.

He told the council's infrastructure committee - which has resolved to raise his concerns with Auckland Transport - the damage was from reversing the direction of water from springs around Redoubt Rd, on the rim of a big old volcanic crater.

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Dr Harger fears earthworks for a four-lane eastern alternative to the Southern Motorway the council transport body intends building from Manukau to Drury to cope with major population and employment growth will divert far more water to the north, killing bush running to the heart of Totara Park and spoiling its walking and bike trails adjoining Auckland Botanic Gardens.

Although he estimates the park has about 139,000sq m of native bush left, mainly in its lower section, he has calculated a loss of almost 10,000sqm in its upper gullies in the first eight years since the earlier project, for which he says there is no logical explanation other than de-watering.

Even attempts to plant more than 100 kauri trees in the middle of the cratered park have failed for lack of water, he says.

Auckland Transport says it has allowed $300 million for the first 8.9km stage of its new highway, which will include widening Redoubt Rd and realigning Mill Rd to Alfriston, but only $172 million for the second stage of similar length to Drury.

It also wants to straighten Murphys Rd, potentially clearing some of Murphys Bush and bringing traffic up a steep hill to a new intersection with Redoubt Rd while cutting through a large homestead and into the rim of the old volcano.

That is instead of a more gentle slope through "greenfields" land earmarked for new houses, as proposed by the Redoubt Ridge Environmental Action Group.

The group says officials have indicated to owners of some of 54 homes facing demolition for the project's first stage that the ultimate cost - past Papakura to Drury - is likely be $820 million over 30 years.

Project director Theunis van Schalkwyk has declined to confirm that figure, saying a clear cost estimate cannot be made until growth plans are finalised.

Designation applications to Auckland Council, expected to be publicly notified in a week or two for a hearing around June, outline plans for two viaducts through mature native bush beside Mill Rd to the south - including 400-year-old puriri and taraire.

But the environmental group, while opposing any incursion into that bush, has opened a new front in claiming Totara Park has been overlooked by the road designers.

"The new road will blast across the top of the gullies - in other words, this is a disaster," Dr Harger said.

Mr van Schalkwyk said an ecological assessment thoroughly investigated what effects the new road may have on bush along the way, but there was no evidence to suggest groundwater changes would have an impact on Totara Park.

If any evidence emerged at a planning hearing or from detailed design investigations, a resource consent would be required to show how that could be avoided or mitigated.

Papakura Marae kaumatua Haare Williams says Dr Harger's findings are "compelling".