Cth Legislation has put in place 'Intestacy Rules' which are rules which determine how your Estate will be distributed if you die without a valid Will. The intestacy rules govern the distribution of your Estate to your next of kin such as your spouse, de facto and issue (children, grandchildren).
If there is no spouse or issue, then provision is made for your parents, brothers and sisters, nephews and nieces, then grandparents, then
uncles, aunts and cousins. There is no provision for distribution of your Estate to relatives more remote than your first cousins. Your in-laws are not classified as next of kin and are not included in the rules for the distribution of your Estate. A step-parent is not next of kin and neither is a mother-in-law or a father-in-law.
To provide a basic overview of how the rules may affect you...
(1) If you have a spouse and no children your whole estate will go to your spouse.
(2) If you have a spouse and children, the first $150,000, plus the family home and contents go to your spouse and anything above and beyond that go to your children.
(3) If you have children but no spouse, your estate will be divided equally between the children. If any of your children are deceased, their children can inherit their share.
(4) If you have no spouse and no children your estate will be distributed in the following order:
your parents in equal shares
your surviving brothers and sisters in equal shares (includes half brothers and sisters) (or if deceased, their children)
grandparents in equal shares
Aunts and uncles (or if deceased their children) in equal shares
As you can see, its better that you just get a simple will together to make sure that your things go where you want them to.
The distribution depend on the state that you reside in. It varies a little stat to state. Somebody, normally someone close to you can apply for letters of administration. They would take care of the details like selling assets etc
If you have a will it is valid until something make it invalid (marriage etc). Just being out of date doesn't make a will invalid it just means a lot of money will be spent sorting it out.