Apologies for not answering. We went away to the bush I had no signal! Hope your hotel managed to assist you...
Try to get Uber driver. Do NOT take a local taxi. Or gautrain to Rosebank mall.
My suggestion would be to use an Uber. They are very reliable, well priced and offer door to door service. The Zoo's parking is currently being renovated in any case so parking is not the most convenient (but there is parking available adjacent to the Zoo). If you don't have the Uber app then use a metered taxi.
Get quick answers from Johannesburg Zoo staff and past visitors.