TCs and HDB
I've been doing some reading on what the duties of the TC are and also looking up the HDB bylaws to see if they extend to animals in the common areas. In general, the TCs look after the common property under Section 18 of the Town Councils Act, with common property being defined under Section 2, and which you can find at Singapore Statutes Online.
As you can see, common property does not include areas within the flats. There are certain limited times when the TC can go onto your property and this is specifically stated - it mostly has to do with pipes, wires and cable (and hence I was wrong about the pipes earlier) but these are very specifically stated examples. In all other cases, the TC must get the permission of the owner, and in even these specific examples, they must give advance notice unless it is an emergency.
On the other hand, the Housing and Development (Animals) Rules refer specifically to cats within the premises. They do not refer to animals in other circumstances.
So why are the TCs saying that the HDB rules do not allow cats in the estates?