Found out from someone in a position to know.
In the morning, bad WX in MEL caused the usual knock-on effects to all throughout the day.
Combine this with 1) the onset of more WX from about 0600z 5pm (cloud down to 500ft at one point, mist & reduced vis), 2) the onset of the peak period with arrivals from everywhere all requiring ILS along with ongoing deps meant metering had to be applied.
Airlines were made aware & reasons, something that often doesn't get communicated down to those at the pointy end.
FWIW those met reports listed above start from 0830z (7:30pm local), by which time things had improved.