If you go to matrix.itasoftware.com and do a search for SNN-PER for dates that you want - click on 'show advanced controls' and for outbound routing code pur EI QF and for inbound put QF EI

Select Premium economy for class of travel and you should get results that give the 1 stop option to Perth

If you then click on the flight option that you want to get the itinerary details and then click on 'copy itinerary as JSON'

in another tab go to bookwithmatrix.com and use control V to paste the itinerary into the box on that page and it should give you an option to book with Flight Network - folow link and should be able to book it online there

Alternatively, if it prices up ok you may be able to book over the phone with Qantas but you will be up for the additional cost for booking over the phone

Looking at the prices, I would suggest that there are much cheaper options for travel to Perth - Qantas coming in at EUR3500+ -- picking for example VN from CDG, ir is offering r/t fares of approx EUR1900 - though you would have an extra stop. It is offering business class r/t for EUR3995

If you are set on QF, it does seem doable, but you could get business class on another carrier for around the same price