The first big rains of the wet season flush all types of rubbish into the river which Katherine draws most of its drinking water from.

It is a double-edged sword for Katherine this year.

After two of the worst wet seasons on record, the region is desperate for a return to a "proper" wet.

And all the weather predictions, boosted by the La Nina forecast, is that it will be any early and good wet.

But for hours, days, or rarely even weeks, the river can be turned into a dirty and smelly soup when bacteria levels peak.

At these times it isn't able to be treated adequately to the required drinking water standards.

"This year, after subsequent wet seasons without good rain and the prediction of La Nina conditions, there is a greater likelihood of organic matter that has built up in the catchment being washed into the river," Power and Water senior manager operations and maintenance, Peter Hopkins says.

"This run-off has the potential to result in poor river water, which would impact the water supply.

"If there is a prolonged period between rain events, the water could be compromised for a longer than usual period of time, which most years is just a few hours.

"This run-off has the potential to result in poor river water, which would impact the water supply. If there is a prolonged period between rain events, the water could be compromised for a longer than usual period of time, which most years is just a few hours."

For example, in 1987, the first big rain came on October 31 with 65.2mm recorded at Tindal.

It wasn't until November 17 the next big rain came along to flush the weir pool of the poor quality water.

This was about the same time they starting using PFAS-laden foam in firefighter training at the Tindal RAAF Base.

Back then, the water plant simply sucked more water from its two bores to keep the drinking water up to Katherine.

Now we know those bores are both contaminated with PFAS and the emergency plant is still small to remove PFAS from enough of the bore water to supply the town.

The new plant is not ready yet, but should be by next wet season flush.

So in a situation like 1987, there would be a fortnight or more where there is not enough guaranteed safe water for the town.

That will mean more water restrictions.

Katherine is already on an odds and evens system for its gardens but it won't be anywhere near enough.

The water authorities have not said what sort of restrictions they will need to the river becomes polluted for days, or weeks like in 1987.

"In these circumstances, our ability to adequately supply sufficient potable water for the town will be constrained and will mean that short term higher level water restrictions would be necessary to preserve the water supply for as long as possible," Mr Hopkins said.

"While we hope this situation doesn't occur, we do need to be prepared and we need Katherine residents and businesses to be prepared as well for this potential risk to the water supply."

By the way, Power and Water said the foam on the river in 2017 was a "natural occurrence" and is not related to PFAS contamination issues.

That flush was caused by 44.8mm recorded at Tindal.

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