The mass appraisal techniques used by the General Valuation Roll has been known to have errors., and the municipal valuation is probably not based on a physical valuation of your property.
The Municipal Property Rates Act (MPRA) does make provision for errors to be corrected through the Supplementary Roll. The onus is on you the property owner, to contest the municipal valuation and the onus is on you to prove that your house is worth less than estimated for the rates role.
The objection needs to be lodged before the deadline on the correct documentation and submitted together with your proof. This is where a valuation on the property is beneficial as the valuation report will have all the information to prove your case and to prove that the municipal valuation is inaccurate.