He said the existing route comprising Redoubt and Mill roads was substandard in both quality and capacity, with a poor safety record of four deaths among 283 crashes in the five years to 2013.
The highway was needed to support new houses and jobs from Flat Bush to the south of Drury and, without it, the corridor was predicted to become too congested between 2020 and 2024.
"The inescapable reality in the current and reasonably foreseeable Auckland environment is that development is going to take place," the lawyer said.
It would also provide vulnerable road users such as cyclists and pedestrians with safer facilities.
Auckland Transport wants to start building the highway by 2020 and possibly as early as 2017 from the Manukau end of Redoubt Rd, but has yet to indicate when it will be able to finish the job to Drury.
That has been queried by Auckland Council consultant reporting planner Peter Reaburn, who after reviewing assessments by other experts, also considers ecological impacts on two bush blocks beside the existing Mill Rd to be significant.
Mr Beatson offered the hearing a preview of Auckland Transport evidence, which says adequate council funding has yet to be approved "to demonstrate financial responsibility for extending the route to Drury."
He acknowledged the clearance of 1500 square metres of vegetation from a privately-tended bush block at 146 Mill Rd was likely to require the removal six or seven large trees, and the topping of others, but said the use of a bridge to carry traffic through the zone would result in "an intact vegetated corridor being retained beneath it."
Another bridge, over a second bush block which Watercare owns at 38 Mill Rd, would be built 13 metres east of an earlier proposed location to retain the densest pocket of mature kahikatea trees in the area.
But steering the new road away from the larger block would have added up to $43 million to the project cost.
Mr Beatson said Auckland Transport proposed 2.2 hectares of restoration planting, meaning seven to eight square metres of new vegetation to every one square metre of canopy trees lost.
Although the Redoubt Ridge Environmental Action Group is also concerned about the project's impact on Manurewa's 216ha Totara Park, he said only three per cent of that area would be required for the road.
Mr Reaburn's report to the five-member hearings panel counted just five submissions in support of the project, and 282 in opposition.
But Mr Beatson said 216 were "pro-forma" submissions and many from people - including a large number based overseas - with little or no appreciation of the local context.
"Most notably, of the 317 land interests affected by the project, only 20 submissions were received, and two of those are in support."
The hearing resumes this afternoon after a site visit by the commissioners, with Auckland Transport witnesses due to be followed by submitters on Wednesday.
