1. Jetstar
2. Scoot
3. Air Asia (if your prepared to do the quick hop over to KL (SIN / KUL / PER fares have been cheaper than direct with the above airlines)
I have also seen cheaper fares on other Asian carries with a stop.
Western Australia has a small population. In additional to all the 'full service' capacity - the market is saturated.
Singapore is funnelling traffic into their hub, for onward travel. They are not and 'end-of-line' carrier. You cannot compare Qantas and Singapore airlines. Different traffic dynamics. You will have customers on SQ with onward travel e.g. Europe. You'll find with JQ / QF et al, it is point-to-point traffic, with maybe some minor regional asia flying on partner airlines.
Qantas (Oneworld / QF FF) will embrace the service from a loyalty benefit.