The ever reliable AFFer Flyerqf informed us in post #13094 that VH-OQD had 'gone tech' (failed) last Wednesday, and ever since there have been delays.
Like you, I don't always do it due to the nature of these blog-type sites, but often a good idea to go 10-15 posts back.
IIRC, an esteemed aviator stated many moons ago that the 'slip' in LHR was for more than a day, so I assume flight deck crew normally have at least c.0630 to c.2100 the next night (i.e. about 26.5 hours) in London at a minimum. Perhaps they have an extra night on top of that? (Anti) fatigue regulations may kick in if there are lengthy inbound delays. QFi (and other operators) must bear the SYD curfew in mind. There'd be numerous other considerations such as how much mandated rest crews at the intermediate SIN stop had enjoyed.
It's eminently possible some staff are off sick with influenza or COVID-19.
Flyerqf and I have suggested these delays will moderate but QFi is trying to do too much with too little 'equipment' (aircraft). It falls down like a house of cards when there's a failure of one plane.
I am in Melbourne where it's cold but sunny (a delightful day!) but while shocking, the weather doesn't seem thus far to have been the major SYD constraint. Of course that may change today given media reports.