You don’t need a literary agent to approach all publishers, but some publishers don’t accept unsolicited manuscripts. Publishers that do accept unsolicited manuscripts are more likely to consider something that has been sent in by an agent.
Research the literary 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. Have a complete and concrete manuscript before approaching the agent.
Australian authors should approach Australian literary agents, because you’re more likely to have success with approaching them.