This is a good move by Qantas. I live in the area and regularly commute to Sydney. The flights might skim a few pax off JQ and VA services (I usually see a smattering of Q-tagged bags on JQ and Virgin flights), but given they both time flights towards leisure pax, I doubt it'll hurt them too much.
REX are the ones who will likely suffer the most as their daily services currently offer the only option for a day-return trip to Sydney (with aircraft sitting on the tarmac overnight at BNK). They charge like wounded bulls on the route, so it'll be interesting to see if their fares come down to compete with QF. Mind you on QF it's a 20 year old rattly turbo-prop vs a 30 year old one on REX with similar in-flight service, so it's not a huge upgrade for regulars on this route. Fingers crossed it's be a success for QF and we'll see 717s and eventually mainline 737s take over.