Welcome to the Bunnings Workshop community @mel93. It's great to have you join us and many thanks for your question.
If there is nothing but weeds in the area then I would start by removing them with weed killer. If your lawn is particularly hard and compacted I would suggest aerating it and filling those holes with Bastion 20kg River Sand. This will ensure adequate drainage in the lawn and will make it easier for the new grass roots to penetrate. Once you have completed the aeration I would apply Hortico 25 Lawn Top-dress Mix and then re-seed or lay turf.