Qantas just has chosen not to offer any discounted tickets in business class one way to Australia.

An airline can charge what it likes. There may be some people who will pay GBP6000 , though most wont. Some companies will have some negotiated deals for corporate travel where they get discounts off of the fare

Fortunately there are plenty of airlines around that are offering decent fares. Luftansa has GBP2011 flying LH LHR-FRA-SIN with QF on to Sydney

I would just not worry about it and just find a decent fare