The council took "well in-excess" of its yearly allocation and has received informal advice that it will likely to have to buy water on the open market to make up for what it took, before it can extract again.

That's if there is water available on the open market.

If not, the council will need to rely on Chaffey Dam's limited supply as it scrambles to secure funds for an emergency water supply plan should the city hit day zero.

TRC water and waste director Bruce Logan said he understands formal advice should be provided by the Department of Industry and Environment (DPIE) or the Natural Resource Access Regulator (NRAR) soon.

"A report will be placed before council's meeting on Tuesday, July 28 to discuss options available regarding the purchase of water on the open market and the related issues," he said.

"The report will seek direction from council on purchasing water.

"If for whatever reason water cannot be accessed from the four groundwater wells, council will need to source that water from either Chaffey or Dungowan dams."

Using the wells would extend day zero by up to eight months if the use dropped from 10ML to 5ML per day.

The cost to the ratepayer to buy water on the open market will depend on the price of water and if any is available.

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A spokeswoman for DPIE said it would not comment on the investigation into TRC but would have to consider an application to pump water.

"Any application to pump additional water by TRC in the future will be assessed under existing processes, including considering the impact of the proposed extraction on the resource and existing groundwater users," she said.

NRAR has confirmed its investigation timeframes vary from up to a month to more complex matters that can take two years to investigate.