I recently tried to find out more about this and while happy to be corrected on any of these points I found the following:
If you are undertaking the act/business of trading properties and these properties are intended to be held for the short-term then any gains are classified as 'trading income' by the IRD and taxed as such, whether you are an individual or a company. If you sold a property that you were holding as a long-term asset one-off then its technically a capital gain and not taxed. The problem, as far as I can see, is the IRD has not released any specific numbers relating to property value or the length of time held as to what constitutes 'trading income' or 'capital gains'. I guess they just "know it when they see it" and tax you accordingly. Once you receive the tax bill, you can appeal and put forward your case - I believe the burden of proof lies with the taxpayer.
The good news is, this has come up before and if you can read past judgements where tax-payers have appealed the IRD's decision and either won or lost.
site:.gov.hk property gain - Google Search
If you have flipped a few properties before, then I don't think you can keep getting away with it - however, if you sell in order to buy a larger flat for your bigger family or something, and the facts support this then I think you could successfully argue that point and not be taxed.
Standard disclaimer: I am not a lawyer/accountant etc.