Ok. So take a look at PC Case Gear and PLE for prebuilt custom PCs as PCCG ships free Australia wide on systems and PLE is local to you in WA.

For the CPU I'd recommend a system with either the Intel 8700k, Intel 8086k or Intel 9900k depending on your tolerance for price. Those CPUs will give you the most overhead for CPU video encoding to Twitch. (That assumes you are buying right away, things will probably be different in late Q2 with Ryzen 3000 launching)

You will need a minimum of 16gb of memory, but 32gb is probably the better option for streaming if you go with CPU encoding.

In a prebuilt, you will pretty much get whatever motherboard the SI gives you, but try and get a high-end Z390 board if you have a choice as the power stages are better on the higher end boards.

As for GPU. Go with either an RTX 2080 or RTX 2080 Ti. These are the two highest end cards on the market, again your tolerance for price will dictate which you get as RTX 2080 Ti's are overpriced. The good thing about the RTX line is the new NVENC encoder built into them provides much better video encoding than the old one. CPU encoding is still better, but if your viewers are comfortable with a lower encode quality you can use NVENC which is almost performance penalty free.

For storage get the most SSD storage from Samsung you can afford. A 970 Evo Plus boot drive will be good but is not mandatory. 860 Evo 2.5" SSDs are the better option though as you get way more capacity for your money. Just buy as much as you can afford. Also, get a WD Red HDD in as bigger capacity as you can for things that don't need to be loaded so quickly. High priority games and OS should be on the SSD. Family pictures and low priority games go on the HDD.

Let me know if you want more information or a detailed build list.