Ensure that your external door has a well-installed deadbolt lock .  If you have a double sided deadbolt on your doors, then congratulations you are off to a great start. Make sure it is a reputable brand and is made to Australian Standards (not a cheap import model).

Make sure when you turn the key when the door is closed that the bolt is thrown all the way to prevent the bolt from being fished back or open easily. Some ways you can check this is by opening the door and turning the key until the bolt is thrown all the way. You then can take a permanent marker or text or and make a small mark on the bolt. Then close your door and then lock the lock and see to make sure that you can see your mark on the bolt to ensure that it is locked securely all the way.