The Limitation Act 1969 NSW in s. 14 states[9]:
           14 General
(1) An action on any of the following causes of action is not maintainable if brought after the expiration of a limitation period of six years running from the date on which the cause of action first accrues to the plaintiff or to a person through whom the plaintiff claims:
(a) a cause of action founded on contract (including quasi contract) not being a cause of action founded on a deed,
(b) a cause of action founded on tort, including a cause of action for damages for breach of statutory duty,
Similar legislation exists in all States and Territories. It must be remembered that a limitation period does not run for a person under 18 (a minor). The six years starts from when the person turns 18 years. Plus there is limited power of the court to extend any limitation, whether for an adult or child. There are similar laws in all states in Australia.
The effect of this is that generally a migration agent should archive files for at least 6 years to cover the contingency of being sued at some stage. If there are matters involving children then clearly the archiving should be longer.  Many solicitors, accountants and medical practitioners archive files for much longer than 6 years.
What do you need to keep records of?