The price tag isn't significantly higher for a bigger pipeline, but the council will build a smaller one that meets demand for at least the next 50 years.

But one councillor has questioned the council's method for arriving at the decision.

Calculations for the pipeline were based on an annual 1.3 per cent population growth, while the projections for a 10-year parking strategy quoted a 2.4 per cent increase.

At Tuesday's meeting, councillor Glenn Inglis said he felt the 1.3 per cent population growth prediction in the agenda was being "a little pessimistic".

"I'm more optimistic about the city than that," he said.

"And, with the carpark we use a 2.4 per cent population growth rate.

"We should probably use similar growth rates in our different reports."

The new pipeline will run between the Dungowan Recreation Ground and Calala Water Treatment Plant.

The one councillors will order can carry between 85 and 90 megalitres each day.

Preconstruction work is already under way for the new $480 million Dungowan Dam.

It's the first dam to be built in the state in close to three decades, and will have 22.5 gigalitres of storage.

That's the equivalent of 9000 Olympic swimming pools.

The dam itself is also being fast-tracked under new planning rules that were announced by the state government, in October last year.

Water NSW is managing the project and has told the council it plans to bring forward construction of the new Dungowan Dam pipeline to as soon as October 2020.

The new pipeline will run to the Dungowan Recreation Ground, where it meets up with the new pipeline from Chaffey Dam, and then heads to the Calala Water Treatment Plant.

The pipeline that's there at the moment has limited capacity and is prone to breaks because of its age.

One of the first key decisions to replacing it, is the maximum capacity of the pipe itself, ticked off the list by the council on Tuesday.

Next, the exact route and pipeline diameter will be determined to meet the 85 to 90 ML per day requirements.

Any major planning requires a look into the city's future. In 'council-speak' that is called a "planning horizon".

A council can choose any planning horizon, but for this project it will look 50 years ahead, because that's considered the lifespan of any large pipeline.

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Part of the reason why the council only went with a smaller line for the next 50 years, is because the use of effluent water and new technologies might reduce the water demands of the city.

It's also because if a pipeline rarely uses its full capacity because growth projections don't come to fruition, it's considered a waste of public money.

From a health perspective, if the full capacity isn't used it can also lead to treatment issues, or slime build up in the pipe.

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