You do not need a literary agent to approach a publisher, but some publishers don’t accept unsolicited manuscripts. Those that do accept unsolicited manuscripts are more likely to consider something that has been sent in by an agent.
Research the agent’s submission style and business practices before approaching them. When you do approach them, be polite and follow the submission guidelines. You also need to keep communication clear and concise. The author should have a complete and concrete manuscript to present to the agent.
Australian authors would be better to contact Australian agents because overseas agents will wonder why you haven’t got local representation.