Premier Steven Marshall says the new measures reflect the increasing risk of virus infections spreading into SA.

"We have strengthened the border in regards to essential workers coming in and I think it's only right," Mr Marshall said.

"As the risk increases, the strength of the supervision increases."

Under the changes, SA residents are no longer allowed to return and tighter limits have been imposed on close border communities, with those residents only free to travel 40 kilometres each side of the state line.

The changes prompted a late rush of people trying to make it back to SA, causing delays at major border crossings from late on Tuesday and into the early hours of Wednesday.

At one stage, police decided to check every truck to ensure it was cleared to enter the state, but abandoned that process as delays increased.

Mr Marshall said the return to random checks was a sensible move.

"SAPOL has only ever done random checks on trucks that come across as essential workers," Mr Marshall said.

"Last night there was a change to protocol for a few hours to inspect all of them and it turned out it was creating unnecessary traffic hazards.

"They went back to the normal practice which has served SA extraordinarily well."

The premier has promised police will have whatever resources necessary to keep the border secure.

"If there is a need to strengthen them further, that's what we'll do but we seek expert advice based upon the risk,' he said.

In other virus measures, SA has also moved to restrict some gatherings with a 100-person limit now applied to weddings and funerals and a 50-person limit to family gatherings.

The new rules follow Tuesday's confirmed positive coronavirus case in a woman in her 20s who recently moved from Victoria to take up a new job.

Chief Public Health Officer Nicola Spurrier said the woman was in hotel quarantine and posed no risk to the wider community.

There were no further cases reported on Wednesday, leaving SA's total since the start of the pandemic at 448.

Only three of those are still considered active.

Australian Associated Press