Let me explain to you how simplistic and perhaps naive your assumptions are.As wonderful as computers are, they don't put 2 and 2 together and add to 4. A transaction of an item or transfer doesn't contain personal information between the parties involved and no IRD may not have the resources to research and investigate everything as they have limited resources.
Let me point out my own example how sometimes they are overwhelmed with work, they really don't have the time. I declared rental income and paid tax for it. The following year they still never sent it their tax form and I had to chase them. In the end, long story shortened, it was me chasing after the IRD so that I can pay tax as if I am so eager to part with my money. So no I do not believe they have the means and resources. So no I do believe they will be able to handle the issues that will crop from CGT once it has been implemented. They may have no bounds where they can go but they would have difficulty allocating resources to check on everyone and everything and knowing what to look for.
To highlight another experience, my property in Singapore which was put up for rental as well, had my property tax, rental income tax deducted from my bank account even before I could say HUH??!!! It was so nice for the IRAS to send me a letter informing me they had made the deduction.
Let me point out that personal information, credit information and spending history are commonly shared among banks more for profiling the customer and marketing rather than nosing around to see if the person has bounced a check in their pass history. So your data is available up to a certain point and though some people have found it intrusive and rather tiresome, their is a limit to how much checks the tax department can do especially if you are not a person of significance as the bigger the fish, the bigger the target on your back. So ordinary people have gotten away with many things by under-reporting on tax expecially those who are in self businesses.