I personally dont think there is margin call clause on pure SGD property loans. Unless the SGD loan is used for financing non-SGD assts, which effectively means there is a cross-currency swap contract tied up together with the loan. Similar to any other financial instruments which are marked-to-market daily, there is a margin call if mark-to-market values falls below treshold.
This risk is not apparent to many such pple as they think they can pay lower SGD interest rates (rather than higher AUD interest rate) on Aussies properties.
If there is no margin call, I would gladly take a JPY loan to buy Singapore properties, where JPY interest rates is rock-bottom low.