To sum up our previous text-message entries, we found out an hour before landing that we couldn't land at Sydney due to fog. Instead we were being diverted to Brisbane.
It's somewhat complicated because different airlines have different rules. The pilot himself came back as far as the Business section and told some passengers that he requires 2000 meters of visibility to land, while other airlines might require less. He said he could land it, but he'd be fired if he did.
There are least three other large planes similarly diverted, including a Qantas A-380 parked beside us for awhile.
After a lot of contradictory and confusing instructions from the crew (take everything, take nothing, no liquids allowed, no liquids over 3 ounces allowed), we deplaned nearly two hours after landing and found our way through typically friendly and low-key Aussie security to a quite pleasant Air New Zealand Koru Lounge.
We find out that the Brisbane airport does have quite a large post-security area. The lounge agent was concerned at fitting everybody in but eventually did cheerfully.
We're here probably at least until mid-day. UA has to fly another crew up here from SYD, they will take over the plane, and we'll complete the remaining few hundred miles of our flight, which was quite pleasant by the way.
The good news is that we don't have to worry about an early check-in at our Sydney hotel.