Three people had tested positive in Shepparton as at Thursday morning after a truck driver passed through the region on a work permit.

MLHD medical director Len Bruce said because the city was in the border bubble, the district had launched a 'border blitz,' with a mobile testing van set up at Tocumwal for the next three days and increased testing at Deniliquin.

"If you are in any way concerned you may have been exposed or you have the mildest of mild symptoms, remember it can be as simple as a fever, cough or a sore throat, please call the Murrumbidgee COVID hotline and arrange to be tested," he said.

Dr Bruce said those who had travelled to Shepparton in the past 14 days would also not be allowed to visit MLHD healthcare and aged care facilities as the district monitors the situation.

"It really is about protecting the vulnerable members of our community," he said.

He encouraged those who had travelled to Shepparton to reconsider visiting any vulnerable family members.

Dr Bruce said the Shepparton cluster, along with Wednesday's 11 locally acquired cases across NSW, were reminders to stay vigilant.

"We need to remember there is still no effective treatment for the virus and there is no proven vaccine ... the only defence we have against this virus is testing, isolation of people that are infected and for us to manage appropriate social distancing."

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He said those who had travelled for the school holidays should be especially vigilant in checking for symptoms, and keep an eye on the NSW Health website to see whether they had visited a place of concern.

For those concerned they may have been exposed who did not have symptoms and had not been told to self-isolate, Dr Bruce said it was better to maintain social distancing and watch for symptoms than attempt to get tested.

He said testing without symptoms could create a "false sense of security" if done less than 13 days after exposure because the virus could still appear after receiving a negative result.

"I think the most important thing people in those areas can do is to practice social distancing," he said.

Dr Bruce encouraged the community to make sure they were registering their contact details everywhere they visited to enable fast contact tracing.

Anyone with the mildest symptoms is encouraged to book a COVID-19 test by calling the MLHD hotline on 1800 831 099.