If it's ticketed LHR-SYD as one flight on one coupon then it counts as one segment. But if you break the flight in SIN (i.e. either you get a different flight number SIN-SYD, or the same flight number but a day or more later) then it's two coupons (and in the second case that is then your entire allowed visit to Asia.

Essentially the restriction is on 16 ticket coupons in the paper ticket sense.