The four main causes
Moving house. Statements, cheques and notices go to the old address, mail redirection lapses after a few months, and the institution eventually marks the contact as invalid.
Changing your name. Marriage, divorce and deed poll changes leave older accounts under a name you no longer use and no longer think to search.
Changing jobs. Each new employer opens a new super account unless told otherwise, and small balances are quickly forgotten.
Death in the family. Estates are settled without anyone knowing about an old account, a share parcel or a childhood insurance policy.
Prevent it in an afternoon
Update your address with every bank, super fund, share registry and insurer whenever you move - not just the ones you use regularly.
Consolidate super into a single fund through myGov once you have checked the insurance attached to each account. Fewer accounts means fewer things to lose.
Keep one list of every financial account you hold: institution, account type and approximate value. Store it where your executor or next of kin can find it.
Use an account at least once every few years. Seven years of inactivity is what triggers a transfer to ASIC, and fees do not count as activity.
Make a yearly habit of searching
Run the free searches once a year. It takes ten minutes to check MoneySmart, the ATO super search and your state register, and money can appear on a register years after you last looked.
Search for older relatives too, with their permission - the largest unclaimed amounts tend to belong to people who have held accounts the longest.
Tip: Set a yearly reminder to run the free searches - new records are added to the registers constantly.
Common questions
Official sources
This guide is based on the official Australian registers and the bodies that administer them. Always search and claim through these sites directly - they are free.