You can get an extension, but not a full 90 day one.
The whole point of the 90 day visa limit is that is the maximum reasonable time (according to them) for a tourist to see what they want, and prevents illegal workers from staying in the country. The US does the same thing, and so do many other countries.
Now, yes, you can exit and come back in, much like the US (although it doesn't reset for the US if you just border hop to Mexico or Canada), however, you're going to be questioned.
- you'll possibly be asked for evidence that you're planning on leaving again soon - plane tickets, itinerary
- you may be asked for evidence you plan on returning home (evidence of a job etc)
Essentially, they're likely going to want to make sure you're a genuine tourist, and not someone who is trying to 'legally' overstay.
If you can show that you're a bonafide Aussie tourist, however, it should be fine.
Since 2014, the laws have changed. Previously you could exit to say, Botswana, and come back in. Now, under the new laws Border Hopping is pretty much banned.
It's still possible, but you'll likely only be issued a 7 day visa:
However, should the previous visa expire while the candidate is out of
South Africa (e.g. visiting Namibia) and they return to South Africa,
they “may be admitted on a new visa for a period not exceeding 7
It looks like now, ONLY if you return to your country of residence, and THEN return to South Africa, will you get a new full visa:
Only if the foreign national re-enters South Africa from their country
of residence will a new visa be issued as per Section 10A(4) of the
This means that foreign nationals who receive their initial 90 day (or
30 day) visa upon entry into South Africa will be allowed to leave
South Africa before the expiry of their visas. If that original visa
is still valid by the time the candidate returns to South Africa,
he/she will NOT be issued a new one, but will be admitted into the
country on that original visa. The exception to this are foreign
nationals who arrive back in South Africa from their country of
residence, or foreign nationals from countries which share a border
with South Africa.