Out of 13 sewerage sites in the Shoalhaven, the two selected were the Ulladulla and Culburra sewage treatment plants.

According to NSW Health's website, testing sites were selected "by a steering committee and based upon areas of concern as directed by the NSW Chief Health Officer".

Shoalhaven Water interim executive manager Robert Horner explained Shoalhaven City Council's limited involvement in the process.

"We aren't really involved other than grabbing the samples and passing them onto NSW Health," Mr Horner said.

"If something is found then NSW Health would make the call on release of information to the media."

Shoalhaven Water staff currently takes samples weekly for NSW Health and doesn't receive the results back.

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The process of taking samples is quite straightforward.

"It really is as simple as dipping a sampling jar into our inlet works at the sewage treatment plant so we're sampling the raw sewage," he said.

Many viruses and pathogens exist in raw sewage, so staff are equipped with the appropriate personal protective equipment to avoid coming into direct contact with it.

"If contact is inadvertently made then we have procedures to deal with this."

Writing for The Conversation Associate Professor of Environmental Engineering, University of Notre Dame Kyle Bibby explained how small traces of COVID can be detected in a sewage sample.

"There's no real standard yet, but most approaches involve concentrating the wastewater sample to some degree to make it more likely that you'll be able to detect any RNA," he said.

"We're not looking for a whole, intact genome, but a small sequence of a single SARS-CoV-2 gene.

"The RNA fragments that we're measuring are too small to physically capture directly. So we use other tricks to snag them, usually by what we call electrostatic interaction - getting the RNA to stick to something like a filter, or using other chemicals to get it to clump together."

According to NSW Health infected people's stools may shed viral fragments into sewage, and shedding can extend for several weeks beyond the person's infectious period.

Viral fragments can also enter the sewer when washed off hands and bodies via sinks and showers.

In September, Kerry Chant revealed COVID testing in sewage had started at some locations from July.

"This is a program that will show us where COVID-19 has been. For instance, we would expect to see viral fragments in Sydney sewage where we have consistently had cases in the community or in hotel quarantine," she said.

"However, if we continue to have very few active cases, there is scope for this testing to provide early warning in places without known or recent cases."