The Lo system
South Africa uses the Gauss Conform projection in 2° belts named after their central meridian: Lo19 for Cape Town, Lo29 for Johannesburg and Pretoria, Lo31 for Durban. Coordinates are on the Hartebeesthoek94 datum, which agrees with WGS 84 to about a metre. Unlike UTM there is no false origin: y is positive to the west of the central meridian and x is positive to the south of the equator, so x is a number of millions of metres.
The converter writes and reads Lo as "y x", the way the Chief Directorate: National Geo-spatial Information (CD:NGI) publishes it, and was checked against its examples for Lo19, Lo29 and Lo31 to the millimetre. Type the belt in front of the numbers (for example Lo31 -2898.70 3304372.90) or pick it in the list next to the result.
Frequently asked questions
Which Lo belt should I use?
The one whose central meridian is closest to your site: Lo17 to Lo33 cover the country from west to east in 2° steps. The converter suggests the belt from the position; surveys near a boundary often keep a single belt for the whole project.
Why are Lo y values negative east of the meridian?
Because y grows to the west. A point 50 km east of the Lo29 meridian has y = −50 000 m; in GIS software that uses east/north axes (the EPSG "Inverted Lo" systems) the same point has E = +50 000 m.