you have to ensure you would have sufficient fund to settle the property if the bank give you less than 80%. The % of your loan would be dependent on the valuation of your property at that time.
If you are a first home buyer (AU citizen/PR), you may be able to get a rebate from the government if your property is less than the set threshold (each state has their owen rules).
If you are an investor, you may be able to get a one-off A$5,000 rebate. Your lawyer/broker should know this and they should be able to help you with the applicaton.