My partner is on the affected QF94 flight.
(Side note: It's a bereavement flight, but traveling on a regular economy fare so the fact that it's bereavement related is probably not on the passenger record anywhere.)

There was no information that anything had happened until about 15 minutes before the scheduled boarding time. Partner was suspicious, as gate number had not yet appeared on the screens anywhere in TBIT.

They were never informed of what had actually happened. Just "mechanical problems." They finally announced that the flight would be "cancelled" around 10:45 (boarding time). Pandemonium at the counters ensued. At this time, all boards and info sources were still reporting an on-time departure, and this continued right up until the scheduled departure time.

People were frantically trying to get rebooked onto the BNE flight, but nobody around was yet aware that the BNE flight had also been cancelled. Because that flight also showed that it was departing on time. I imagine a similar scenario was unfolding over in T4, and everyone there was trying to get rebooked onto the SYD flight without knowing that it was also cancelled (or, rather, significantly delayed).

Finally, everyone was told that the flight was going to be delayed until 10:30am tomorrow. They were to head out of the departure area, pick up a letter, and get onto a bus for overnight accommodation. As they were walking out, the status finally changed from "on time" to "delayed." The letter stated only that meals and transportation would be provided by Qantas, but they were verbally told that accommodations would be provided.

When they walked out of the airport, there were news people and police everywhere. This was the first indication that something very strange had happened, and this is when they actually found out about the collision involving both the MEL and the BNE flights.

They are currently on the bus heading to the hotel.
We are checking on what our credit card insurance will cover for a situation like this. Looks like $200 for meals and accommodations, so not sure how much of that will be used since Qantas is paying for some or all of this. They were not given their luggage back, so it would have been nice for toiletries to be included in the card insurance, but apparently that's only covered if the luggage is delayed on arrival.

I'm pondering whether it would be better to get another route instead of the LAX-MEL direct flight. They are saying the flight will depart at 10:30am (7.5 hours), but I am very skeptical given the nature of this problem.

UPDATE: My partner's group of passengers have arrived at the hotel. They are still maintaining that the flight will go tomorrow at 10:30am, as it has not been declared unserviceable.