Whenever you have less than 20% deposit, you will almost always have to pay mortgage insurance or Lenders Mortgage Insurance (LMI). As the name suggests, this insurance is a one-off payment by the borrower to the lender (or lenders' insurer) to 'insure' the loan.
It insures the lender for any short fall on a loan, so if you were ever in the position where the lender took legal action and forced you to sell your property, it would generally cover the lender if there was different between what your property was sold for, and any loan amount still owing after the sale.
On 100% loans, dependent on the lender and the risk, mortgage insurance can cost up to 3% of the amount you are borrowing. Up to 95%, the amount would typically be up to 1.2% - 2.0% of the loan amount. As you get closer to 80%, the cost can discount substantially.