I don't know how severe your back problems are, I was "grounded" for almost four months because of back problems. Even F was a torture.
There are just two ways open for you to solve the problem:
Call the airlines to book you on a flight that has very few passengers and tell them of your problem. If you are lucky, there might be a whole row empty in the back and you could then lay down, but OZ flights are usually quite full. No solution for the short haul.
The other is to get to your doctor and have him make an injection the day you fly and ask for a prescription in order to get another one for the next flight. This has worked for me on occasions. Remember to get enough rest once at destination and do not make the mistake of overdoing it because the pain is not there any more.
Good luck