What is the ruling on purchasing a house through banks for someone who lives in Australia, has five children, and finds it difficult to rent a house?

It is permissible for the bank to purchase the house for itself and then sell it to you in installments. This is called a Murabaha contract. However, if the bank were to lend you the value of the house and then you would repay them more than that amount, this is precisely usury (riba), and it is not permissible except in a case of dire necessity that cannot be averted through a permissible means.