I will be taking a one-month trip departing from the airport in Johannesburg, South Africa and need a secure place to park my car. I see that the airport has long-term parking, but is it safe to leave my car there for one month? Are there are any other options for long-term secure parking in Johannesburg?
It's worth noting that one reference on the Lonely Planet forums reckons the South Gate parking is under shade netting - which in Joburg's hailstorms may not offer the protection you desire.
However, your link would seem to indicate that's only sections P5 and P6.
Another option would be to rent a storage garage big enough to fit your car. They have 24 hour security, and there are some near the airport.
Finally, you could consider Omni Park - who manage several car park locations in the Cape (I realise that's a different area), and may have contacts or branches in or around Joburg.