Might be a short flight but I can't actually sit for 3 hours with my foot flat on the ground without reasonable discomfort. And not enough room under the seat in front to rest boot on its heel - which is the position I need it to be in.
I've already moved myself into J for outbound but inbound is A330 so Y only.
I'm 5'7" I'm thinking you might be taller. I'm guessing you're not driving when you land so I'd pop a lot of pills prior to the flight. My uncle has to use compression stockings, could you get one of those to help lessen the potential swelling and discomfort? Maybe ask at the chemist?
I'm guessing you mean A320 on return, so head down the back of the plane, I reckon better chance of seat free next to you although if it's the last flight of the day to Syd they can get full depending on the day you're flying. Swap to earlier or next day if seat map looks full? I've done that on the Air NZ site before and cost maybe $1 fare difference with $30 change fee I think but I'm not sure how that works if you bought via VA.
Could you swap to 2 flights with PE there and back rather than J and Y? or is it a 767 without PE?
If the above doesn't work and if you think you will fly that route again and you bought via VA do they let you travel bank it to use some other time and you could either fly EK J to SYD or route via BNE or MEL on a widebody with PE then to SYD on a shorter 1 hr flight? You'd get more sc's!

That's if time allows.
Put on a bit of a painful face at check in just saying it's painful having it down and you're hoping for a short flight etc, but not too painful where they say - you can't get on the flight you need to be checked by a doctor and then you miss your flight! Just to increase chances of maybe a seat spare next to you.
They should be able to assist with airport wheelchairs as there always seem to be people in these. There was an article in the news of someone being let down by Air NZ but statistically speaking you should be ok with that side of things.
Those are things I would do if I were in your moon boot!
And pills!
Just had another thought, if it's your left leg then try for an aisle seat on the right side of plane A320 and vice versa (I think I have that the right way round), but then you can stick your leg out into the aisle when people aren't going back and forth. No falling asleep though or someone will trip over it and that will cause you pain!