The expansion will indicate if there is someone in a community with COVID-19 so action can be taken.
The expanded surveillance will start on Wednesday, following a strong response to the testing in high-priority areas.
The testing encourages people to be tested for COVID-19 even if they have no symptoms, and is designed to give clearer insight into whether COVID-19 is present in a community.
WNSWLHD Chief Executive Scott McLachlan said surveillance testing had a stronger role to play given that COVID-19 can be present with very minimal symptoms, particularly in children, and people who have been vaccinated.
"Surveillance testing has a particularly important role to play in communities where we have seen detections in sewage in towns where there are no known cases, or where we haven't been able to identify a source of transmission," he said.
"But with vaccination rates across the District are increasing at a rapid rate and as more people build that protection there is potential for less obvious or minimised symptoms.
"So widespread surveillance testing can offer greater assurance for our communities by testing as many people as people, some who may have been hesitant to come forward previously.
"We saw a really strong take-up of surveillance testing in areas like Cowra, Trangie and Dubbo where it was initially piloted. In Cowra alone the number of people being tested has increased by hundreds each day.
"Surveillance testing is different from what we've been doing previously in that we're encouraging people to come forward regardless of whether they have symptoms or not, we want to test as many people as we can.
"People who aren't close contacts or who don't have symptoms themselves or in anyone else in their household are then not required to self-isolate until they get their results."
Mr McLachlan said the process for testing across WNSWLHD will not change and encouraged anyone showing any signs or symptoms to continue coming forward for testing as a priority.
"We obviously still need those people showing any signs or symptoms, or those identified as close and casual contacts, to be tested immediately. In those cases, people will still need to isolate until they receive their results or in some cases for the full 14 days regardless of their results," he said.
"Widespread testing is still one of the most effective ways we can catch COVID-19 cases early and stop the spread of this virus.
"I'd encourage anyone who comes forward for testing to let our staff know what their circumstances are and whether they are symptomatic or an identified contact, to make sure you are following the right advice for your situation.
"It's also important people continue to monitor the list of venues of concern where transmission may have occurred or there is an exposure risk to determine if they are a close or casual contact, then follow the health advice accordingly."
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