The Shop Distributive and Allied Employees Association has urged people not to visit the shops after having a COVID-19 test.
"Once you're tested, don't put others at risk by going shopping," SDA state secretary Bernie Smith said.
Mr Smith said while the union has not received any reports of this behaviour in the Southern Highlands, he urged everyone to follow the rules.
According to NSW Health, people who have been tested must self-isolate while they wait for their results.
The union president has encouraged NSW health minister Brad Hazzard to display signage at all shopping centres to ask customers not to come in after COVID-19 tests.
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In addition to this, the SDA has set out a 10-point health and safety plan for businesses to protect workers from COVID-19 exposure.
Recommendations include contactless payments, the installation of Plexiglass screens at cash registers, social distancing signage and an adequate supply of hand sanitiser, gloves and personal face shields to workers.
Companies should also provide free bags or make customers bring their own, increase security at stores, sanitise workstations and equipment regularly and take a zero tolerance to abuse.
Southern Highlands Chamber of Commerce and Industry chairman Steve Horton stressed the importance of staying home.
"If people are not feeling well and they're being tested, they need to stay home," he said.
"We can't afford to be shut down. It only takes one person."
Southern Highlands small businesses have had several challenges this year, including COVID-19 restrictions, the bushfires and drought.
Destination Southern Highlands issued a statement on Wednesday night to remind the public the area was still open for business.
"The Southern Highlands remains open for business and no formal lockdown or hotspot advice has been received from the state government for our shire to date," the statement read.