Yes, JQ only do MEL- SIN 2 times a week, but prior to around Oct 2017 it was 6 times a week.
It was always full, in both classes. As the route structure to LHR was changed, QF stepped in with a daily 380 MEL-SIN, and JQ went to 2 times a week.
The same thing happened with MEL-NRT. JQ fly the new/marginal routes, if the business case stacks up (i.e. good premium load projection), QF take it over.
A nice way to limit the losses on new routes, get slots and establish a presence in the new destination with code share arrangements in the destination country.
SQ do the same with SCOOT.