Individual taxpayers must be registered with SARS if their income partly or wholly originates from rental-, business- or interest income.
If an individual earns a salary below R120k per annum and contributes towards a medical aid- and/or retirement annuity fund OR the salary income exceeds R120k per annum, then the individual must be registered for income tax as well.
In fact, every salary- or wage earner within South Africa must be registered for income tax as from 2011 tax year as this is a requirement for allowing SARS to approve IRP5's of employers and pre-populating the information on SARS E-filing.
Note, that the Tax Administration Act of South Africa does allow SARS to levy monthly penalties against any return that is outstanding.
SARS therefore would know if an individual taxpayer must legally submit an income tax return or not by virtue of the information it receives from employers and other third parties.
SARS also does not issue income tax returns automatically anymore, therefore individual taxpayers should have a tax specialist looking after their interests and tax affairs or else there could be a nasty penalty awaiting a tax client when he/she wants to submit a return for the first time.