Qantas have been putting QF code on flights operated by J* for year, what are you worried about?
If you are booking out of the UK, travelling to Christchurch, Qantas will put some SYD-CHC legs, operated by JQ on the itinerary. The UK passenger or their Travel Agent will have to look closely to see that the flights will not be Qantas flights. Perhaps by the time they get off at SYD, they are too tired to notice which plane they are getting on next.