But Queenstown Lakes Mayor Jim Boult says the project’s scope has increased, and the new estimate has been reviewed by an independent consultant.
It now featured larger diameter piping, higher-spec joints and valves at each end and would connect with a new pipeline in the Hanley’s Farm subdivision, extending its length by about 200 metres.
The original scope of the project, estimated to cost $5.3 million, had "no future-proofing", Mr Boult said.
The council had also become aware of "significantly greater" housing capacity on the southern side of the Kawarau River through higher building density.
"It wouldn’t have been too many years before we would’ve been faced with putting a second pipe in to add capacity or to replace it."
The council was playing catch-up with infrastructure investment and had to "get ahead of the game".
"What we’re doing is fronting some additional cost, but we’ll save a considerably larger sum of money at a later date."
The independent review had eliminated the suggestion contractor McConnell Dowell had loaded up the estimate with cost overruns from the bridge.
Mr Boult said the project as originally scoped would have left the council with a pipeline under the road with no connections at either end.
That would have meant the new road on the bridge’s southern approach would have had to be dug up later to connect the pipeline to housing developments.
Asked if ratepayers would be surprised that a pipeline could cost one-third the $22 million price tag of the new bridge, he said it was a "complicated piece of engineering".
"When this pipe is full of water and solids, there’s a lot of weight in it, and even the hangers underneath the bridge are a fair piece of engineering on their own."
Although councillors approved the $7.2 million estimate on Thursday, they had also instructed staff to "go back and negotiate hard" with the contractor.
"So it’s not going to cost any more than that."
The council would claw back a significant amount of the cost through future development contributions, he said.
Pipeline work is expected to start within a month.