In the Regulations made in terms of Section 27(2) of the Disaster Management Act published on 25 March 2020, it states that all persons performing essential services, obtaining essential goods or seeking medical attention, may be subjected to screening for COVID-19 by an enforcement officer.
The Lockdown Regulations can be found here (25 March), here (26 March) and here (2 April).
Please visit www.gov.za/coronavirus for regular updates to the Regulations.