Obviously the details about why matter, not necessarily the length of delay itself. HKG has been slammed with extreme weather the last 2 days and the airport has experienced significant delays as a result. As other posters above had said, if it's weather related you're not really entitled to compensation....
I'm not sure if your original SYD flight got caught up in the weather issues I'm referring to. I didn't know the curfew thing but the poster above mentions a plausible reason why things cascaded.
As to why this flight and not that one for rebooking, who knows but I know CX ground agents and team responsible for reaccomodating everyone on different flights have been at battle stations the last 48 hours. Sounded like hell for the CX staff, but they seem to have done a fairly good job overall. They might have more than a thousand misconnects or cancellations the last 2 days so my guess is they're just doing their best, follow up might help your case.
I'm sitting on board a CX flight delayed "only" an hour into Hong Kong. We landed a few minutes ago. We have since been advised we don't have a gate and will experience further delays. A group of angry connecting (I assume) passengers were apparently raising their voices at the flight attendants about misconnects, the captain has since come on the intercom and asked everyone to chill out and explained the weather has been a big problem the last few days. Lol and he asked everyone not to take out their anger on the flight attendants, because all flights here at HKG in and out are affected.