Its a common complaint on old ADSL copper plans that rain fills up the pits and exposed copper wires get affected. FTTC is actually just ADSL except that its fibre (unaffected by rain) up to the pit outside your front fence. So if you are getting issues then the rain is affecting cabling between the pit outside your fence and up to the house.
First thing is to quantify this slow down. What do you normally get on speed tests? When it rains what do you get then? Once you've tested a few times then get Optus to request a NBN contracter come out and inspect your pit and wiring to the home. They might find a possble fault. The NBN is responsible for the signal right up to the front of the house.