Hi @enap,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
My suggestion is to start by dethatching the lawn. Check out How To Dethatch Your Lawn for some guidance on the process and the tools required.
Once the thatch has been removed, you'll want to aerate and topdress the lawn. Check out How To Aerate And Top Dress Your Lawn for some guidance on the process and the tools and materials required.
You should then water the lawn regularly to promote growth and mow when required. Mowing will train the existing lawn to put its energy into sideways growth which will help fill out the area.
You might also like to check out A Seasonal Guide To Lawn Care for some tips for lawncare this Spring and Summer.
Let me know if you have any further questions.
Jacob
