It says the bridge, 30m long and of the same width, will require clearing 1500sq m of bush amounting to just 3.6 per cent of the block.
It also wants to build a 100m bridge over bush and scrub owned by Watercare further north at 38 Mill Rd, after following consultant ecologist Dave Slaven's advice to keep it clear of the area's densest pocket of mature kahikatea trees.
Mr Slaven said in earlier evidence that the bridge over the Cheesman property would require the loss of six or seven large trees such as kahikatea and puriri well as "a few other less sizeable canopy trees."
But he said replacing the existing tree canopy with the bridge would not mean a substantial loss of sunlight for surviving vegetation.
That was challenged yesterday by a consultant ecologist for Forest and Bird, Nicholas Goldwater, who showed the commissioners slides of bush through which sunlight shone through the canopy.
Although the bridge would breach the narrowest part of the Cheesman property, that was a deep gully which constituted "core" bush - one of the best examples of taraire-puriri forest he had seen in the Auckland region.
"Some of the trees within the trees within the footprint of the proposed bridge are likely to be over 200 years old," he said.
"There will be a permanent loss of high-quality habitat for indigenous birds and potentially long-tailed bats and lizards."
Auckland Transport lawyer Andrew Beatson earlier urged the commissioners to give "very little weight" to Mr Goldwater's evidence, saying he had prepared a report for the council body in 2013 stating that potential adverse ecological effects had been significantly minimised by careful route selection.
But Mr Goldwater denied tailoring his evidence to suit his clients, saying he was one of four authors of the report and did not assess the Cheesman property for it.
Although he proposed lessening impacts on the bush with a higher bridge, Mr Falla said the highway should be moved as far west as Mill Rd.
Auckland Transport says skirting the Cheesman property will add up to $43m to the project cost, and a higher bridge will have large impact on the surrounding landscape as well as being more expensive.
Forest and Bird board member Lindsey Britton said Mr Cheesman intended bequesting his property to integrate it with an 8ha bush block 100m to 200m to the south which her organisation had owned since 1975, and hoped ultimately to form part of a green corridor between the Hunua and Waitakere ranges.
New highway:
Four-lane road running parallel to the Southern Motorway, from Manukau to Drury via Alfriston.
First stage: 8.9km to Alfriston, including upgrading southern 1.8km of Murphys Rd, as well as widening Redoubt Rd and replacing Mill Rd.
Cost: $297 million.
Property impacts:
64 homes and a motel to be demolished; parts of about 250 other properties to be acquired.
Construction timetable: Starting from Manukau between 2017 and 2020, and finishing after 2026.
Timing of rest of the 18km route to Drury: Unknown.
