According to the Road Safety (Vehicles) Regulations the minimum standards for car registration in Victoria is 'S' (180kmh), or the maximum speed of the vehicle, whichever is the LESSER.
So unless the car is not capable of 180kmh, a vehicle fitted with S rated tyres is still roadworthy (but not necessarily covered by all insurance policies, even though I'd argue they should be).
And yes I know it doesn't make any sense because that means a Porsche capable of 250kmh could be fitted with S rated tyres, but that's what the regulations say.... And the speed limit is no more than 110kmh anyway