A Queenstown lobby group battling the Government's highways manager over a planned $25 million bridge says the latest assessment has been bungled.
The group has vowed to fight on but the New Zealand Transport Agency is pressing ahead with its plans.
Tenders for the work close at the end of the month and it is hoped construction of the bridge over the Kawarau River will start by the end of the year.
Late last week, the agency publicly released its assessment of two bridge options over the Kawarau River: its preferred site near the existing, one-lane bridge and a ''Boyd Rd'' option, about 3km downstream.
A report from Abley Transportation Consultants said the curved, 250m Kawarau Falls bridge had 50% more economic benefits for 44% less cost.
Lobby group member Adam Childs, of Arthurs Point, said the agency's assessment has used the wrong route.
The agency's route turned west into a residential area, rather than east to connect with a new eastern access road, past Queenstown Airport's main runway, he said.
Mr Childs also criticised the report for using out-of-date data and excluding potential benefits.
''There is reasonable doubt about the wisdom of locating the bridge as proposed.''
Agency planning manager Tony Sizemore said the model did include the eastern access road - but also allows traffic to flow west towards Lucas Pl. Traffic volume plots in the document confirm this.
The agency's acting southern business unit manager for highway network operations, Ian Duncan, said the assessment - prompted by a request from the Queenstown Lakes District Council - did not derail its tender process.
He described the assessment of a Boyd Rd option as a reality check ''to ensure we didn't miss something''.
Mr Sizemore added that the agency had a social responsibility to answer valid inquiries from a well-organised group.
He said the agency did not want to discredit the group - but they were not traffic or road engineers and any Boyd Rd comparison had to be done with no detailed cost estimate.
The Boyd Rd route is estimated to be 1.65km-long, with a bridge spanning about 200m.
That compares with the 250m-long curved replacement bridge planned.
The Abley report estimated a new Boyd Rd bridge might cost $36 million.
Mr Sizemore described traffic modelling as a specialist art form.
Queenstown's traffic was difficult to understand because of seasonal variations and changes in origins and destinations.
He said a ''winter model'' was used because Frankton's roads are busiest during the evening winter peaks - probably because of skiers and boarders returning from The Remarkables skifield.
The benefit-cost ratio for the replacement bridge is now considered to be 1.5 - which means the cost is outweighed by economic benefits such as time savings, reduced vehicle operating costs and congestion benefits.
That is higher than the 1.1 ratio publicised in 2011.
Mr Childs' assessment put the ratio for a bridge connected to the eastern access as 3.3 - with the caveat he was not a traffic engineer.
Mr Sizemore said he was provided with Mr Childs' report on Friday but had not yet read it.
Tenders for the design and build contract close at the end of the month. Mr Duncan said he would like to hand the site to the successful tenderer by early September.
Construction might take two years.
There is already much road-building in Frankton with two new roundabouts on State Highway 6 at Grant Rd and Glenda Dr, and work on roads in the Five Mile and Shotover Park developments.
Mr Duncan said the agency would work with the local council on long-term transport solutions.
