Yes, winter weather can cause leaves to discolor and drop. How cold does it get there in winter? I would think Queensland would have similar weather to ours here in Florida, but we do get temperatures below 5deg. C at night in winter and occasionally (rarely) down to 0deg. sometimes.
If your nights are getting cold, your ixoras are probably dormant right now. You need to slow down or stop watering altogether, and don't fertilize either. They really can't metabolize much water or plant food right now. Don't let them dry out completely, but be very sparing with the water.
Another month or so, you should start seeing healthy new growth begin and that is your indicator to give some fertilizer (but a very light dose for new-ish plants) and gradually increase watering as the weather warms up. You can tell if there is too much fertilizer, as the new leaves will develop brown, burnt looking edges.
