You'd know, but by my maths (using the Australian term!), I agree, because the flight crew probably signed on at about 0235 AEDT this morning in SCL, and reaching SYD by 2200 allows IIRC sign off at 2230, which would be 20 hours - that is, the normal 18 hours plus the 'by agreement' extension of crew duty hours to 20.
Do they "pull the pin" (i.e. terminate in MEL, and spend overnight at an hotel in Greater Melbourne) if it looks like being a few minutes over 20 hours, or do they "have a go" if regulators will permit that sort of approach to the last few minutes? I assume this is up to the Captain to determine and not for management in what one of our aviators amusingly referred to as 'where they (operations centre) play a video game'.
With so many other flights disrupted, airspace along the MEL - SYD track (and on approach) would have to be busy late tonight. That slows flights down and mitigates against any improvement in timings.