Assuming AUD430k property with 70% loan AUD300k. You took the loan in SGD hence liability is SGD390k, assuming exchange at that point in time is 1.3 (AUD300k/1.3 = SGD390k)
Now exchange rate drops 1.0 i.e. AUD weakens against SGD, your SGD liability now becomes AUD390k (SGD390k / 1 = AUD390k).
Your LTV ratio has become 91% instead of previously 70% (AUD390k / AUD430k). That triggers the margin call from the bank, to bring the LTV ratio to lower levels