I'll be at the outside drinking area of the Harbour View Hotel from noon onwards, wearing my Akubra (if you don't know what that is - essential Aussie knowledge - Google it!) and a red backpack. Based on last year's experience, there's usually a barbie/sausage sizzle going on the pavement outside and it can get busy around there, but I reckon there will be plenty room for a few more, if not outside then in the main bar.
Yes, the Rocks district does get busy on Australia Day but the area round the Harbour View is quieter and more open - there's plenty of room on the grassy area under the Harbour Bridge to spill over onto - though we've always found plenty of room within the pub - it has several separate bars, including an upstairs room with great views.
Worst case scenario, if things get really bad we can always move on to the Hero of Waterloo, the Palisade or one of those other heritage pubs within staggering distance.
The trains will busier than a normal Saturday on the 26th but I've never found them to be that bad - but then I commute through the CBD every day and am used to the rush hour.
The pub is at 18 Lower Fort St, right underneath the southern side of the Harbour Bridge (that coathanger shaped thing).
Map is
here.