Check this website out. https://www.bonsai.za.org/2009/10/22/information-on-acacia-burkei-black-monkey-thorn/
I don't have any experience on Acacia's, but this what I did for a plant with a similar situation.
I remember one time my lucky bamboo plant(Dracaena sanderiana) was having health problems. The leaves one by one started turning yellow. Then a bad odor would emit from it's pot. I lifted it from it's pot, and good lord, it had the most terrible smell. Many of it's roots were slimy and black. I took a pair of scissors, and trimmed away the blackened roots. Thankfully there were a few white tipped roots left. After a few weeks of babying it, it bounced back to it's former good health.
So here is what I suggest for you;
Trim away any black, diseased, or roots that don't look lively. You may want to re-pot in a new batch of soil. The old soil might still be harboring whatever it is that caused the root rot, and could be a contributing factor to it's declining health.
I'm not sure about how your tap water is in S.Africa, but if chlorine/fluoride, or any other chemicals are present in the water I would stay away from that. And if you must use tap, let it sit out for 24 hours, and boil it to remove the chlorine. Bottled water is a good substitute too.
Watering should never be on a scheduled routine. Water only when the soil is dry to the touch. Do not constantly keep the soil sopping wet. This leads to suffocation of the roots, which in turn leads to root rot, fungal diseases, or other nasties.
Fertilization is not something you want to do all the time. Many fertilizers can burn roots when over used. I would suggest doing this only in the spring growing season.
Goodluck on your Acacia. Post pictures, and keep us updated.