It depends on the needs of your business. There are a couple of things to consider:

• The contracts you work on. Some contracts will require a minimum amount covered. For example, if your business is within a shopping centre, you may find the contract specifies $20 million be covered.
• The conditions of your licence (if applicable). As an example, electrical contractors are required to hold a minimum of $5 million public liability cover in order to obtain their licence in some states.