Normally, your father's naturalizing in South Africa (or anywhere) while you were a minor would have caused you to lose your Dutch nationality along with your father. However, I believe that this would not be true if you were South African from birth, because you would not have been acquiring South African nationality at the time of your father's naturalization.
If you were born in South Africa, however, have probably lost your Dutch nationality after ten years of residence outside the Netherlands. The ten-year period starts at the age of majority, so in your case you would have lost your Dutch nationality on your 31st birthday, except that the law was changed in 1985 and your 31st birthday fell in 1987. With the change of the law, a new 10-year period started, and the rule was a bit different: you would have lost your Dutch nationality only if you lived in South Africa for ten years. Therefore, unless you lived outside South Africa for part of the time between 1985 and the end of 1994, you would have lost your Dutch nationality at the end of 1994.
If you did live outside South Africa, you would have delayed your loss of nationality until ten years after your return to South Africa. If you delayed it until after April 1, 2003, then you have to look at the new rule that took effect on that date, which provides for loss of nationality for anyone with dual citizenship who lives outside the Kingdom of the Netherlands and the EU for ten years. If you retained Dutch nationality until April 1, 2003, you probably lost it on March 31, 2013, depending on your residence history.
There is a provision for former Dutch citizens to regain their Dutch nationality through a simplified ("option") procedure, and it seems that you are almost certainly a former Dutch citizen if you are not now a Dutch citizen. However, another requirement is to have been living legally in the Netherlands for at least one year at the time of your application, so you would need to find some basis for a residence permit before you can apply. The Dutch Immigratie- en Naturalisatiedienst has a page on the option procedure, where you would fall under the fifth point:
You are an adult. You are a former Dutch citizen. And you have been living in the Kingdom of the Netherlands for at least 1 year. In this year your have had a permanent residence permit or a temporary residence permit with a non-temporary purpose of stay. Citizens of the EU/EEA or Switzerland do not need to have a residence permit.
Disclaimer: there are some exceptions to the ten-year rule that do not apply to most people, but could apply to you. My knowledge of Dutch nationality law is that of an interested lay person, not of an expert. You may want to consult a lawyer with experience in these matters, and you may at least want to apply to your Dutch consulate for a formal determination of your status with respect to Dutch nationality.
