I'm flying to South Africa from New Zealand for a holiday and I would like to know if I can buy a one way ticket there and a one way back as the flights are slightly cheaper and better
Yes, you can, but rarely would two one-ways be cheaper than a single return, so I suggest you start by doing more research.
If you're planning to buy the one-way back to NZ at SA then you should consider that it's possible (though improbable, in my experience) that you'll be asked to present your return/onward ticket at some point on your way to SA. That point is unlikely to be SA immigration. It will most likely be at the airline counter (especially if you're flying Air NZ, which are known sticklers, to put it politely). A good workaround is to get a friendly travel agent to reserve (not buy) an onward ticket for you (at a different airline, to a different country) at no cost. You will be able to show the reservation and be let through. The reservation will then expire a few days later.
,This trick is known to the airline and therefore a one way (OW) is never a 50% of a round trip (RT) but usually 70-130% (yes, sometimes even more then a RT).
I can't see how 2 OW be less expensive then a RT (same airline?!) but assuming you're right - yes you can do that; it's perfectly legal from the airline POV.
The only trick is that you should make sure that you buy them at the same time since your return flight may increase in price at destination (most likely it will).
Anyhow, I would like to know, after your return, that this strategy was less expensive then RT. Good luck