Welcome to Bunnings Workshop.
What sort of area (m²) are you looking to cover?
First off, the substrate under where you place this turf, needs to have good drainage as a priority and it also needs to be flat.
If you could please post a picture or two of the area that you're looking to cover, that'd give us a better idea of what you're taking about.
There are heaps of people here that can help you out, myself included, we just need a wee bit more info from you.
It's great to have you join us, and thank you for sharing your question about what to put underneath your fake grass.
It's great that you've received excellent advice from @MikeTNZ. If this is the first time you've laid artificial turf, I recommend thinking about drainage and water flow. If your area is not prepared properly it might cause unintended water pooling or water flowing towards your house instead of the drain. We look forward to seeing the photos. If you have any other questions, please don't hesitate to post them. Our members would be more than happy to give you advice and information should you need it.
Here is a handy step-by-step guide: How to lay synthetic turf
If you need further assistance, please let us know.