It varies by state.
NSW it's $3000 (+ $1000 per extra adult, $500 per extra child) Children under 3 not charged.
SA it's $3000 (+ $1000 per extra adult, $500 per extra child) Children under 3 not charged.
WA it's $180/day + $60/person, so $2520 ( + $840 per extra person). Children under 6 not charged.
QLD it's $2800 ($910 for extra adult, + $455 for extra child).
No international arrivals to NT right now, but they charge $2500, and $5000 for families (i.e. 2 people or more in the room),
Good question about flight being cancelled. We had some award flight into SYD on SQ during December, booked before the charge was brought in, that was cancelled this week, we've maintained the PNR with various waitlists, and we might ask if we can add revenue flights to the PNR instead. I am not sure what the approach is to cancelled flights.