Having a look at Rex's proposed SYD-MEL schedule. They will start in March with 3 Boeing 737-800s and plan to operate 9 return flights per day.
I just had a look at how the fleet utilisation would work. Each plane will fly 6 sectors per day, with 55-60 minute turnaround times between flights. Those turnaround times are better than the industry average, which will help the schedule to absorb delays during the day. But if one of those 3 jets goes out of service, the whole operation turns to chaos. These are relatively old 737s, so I'd say there's a reasonable chance this will happen.
So when Rex's Deputy Chairman tells the media that Rex is going to offer "superior reliability", I just don't see how this is backed up in fact.