SA has had 443 known cases of coronavirus and no community transmission for more than three months, but concerns remain some cases could be going undetected.

SA Pathology's clinical service director Tom Dodd says it is possible other people have been infected but are asymptomatic.

"We think the number will be very low (but) that's important to know for the planning of ongoing surveillance programs," he said.

In a one-month trial, surgery patients at the Royal Adelaide Hospital can choose to have nasal swabs and blood samples taken while they are under general anaesthesia.

The tests will tell if they are infected or if they have antibodies suggesting an earlier infection.

The trial comes as SA prepares to close its border with Victoria in response to the large rise in COVID-19 cases in Melbourne.

The measure will prevent anyone entering the state from Victoria from midnight on Wednesday night while people living in cross-border communities will be restricted to a 50km radius inside SA.

Police Commissioner Grant Stevens said maps were being produced to clearly show the entry limits and anyone from Victoria found breaching the rule may face a $1000 fine or further action.

People living near the border and needing to enter SA to conduct daily activities, like attending school or work, must have a pre-approved essential traveller exemption.

Those residents who have an existing approval under the previous directions do not need to reapply.

"It will enable them to undergo those services and functions they require as part of their normal daily lives but will prevent them from travelling too far into SA," the commissioner said.

Mr Stevens said additional resources would be deployed to the state's crossing points, with some smaller roads being physically blocked.

SA has been operating an online pre-approval process for people coming from interstate and so far, there have been about 24,000 applications.

Under the new arrangements for Victoria, some people using the online process may be turned down.

SA Police are also in discussions on the possible use of Australian Defence Force personnel to assist with the border lockdown.

Health Minister Stephen Wade said the restrictions would allow health officials to respond to any new infections.

"If there is an infection that's a result from cross border travel, we need to make sure our society can respond to that," he said.

SA reported no new virus cases on Wednesday and has only two active infections.

Australian Associated Press