This isn't a forum for medical advice, but I think I can make two suggestions. The first is to show your pack to your doctorwith you attached to it. He or she might be able to make suggestions as to its suitability for what you want to do.
In terms of the pack itself, any pack that does an effective job of transferring weight to the hips is going to be the best bet. That may simply be a case of adjusting the pack's straps. Or, it may be a matter of buying a pack that does this job for you. Traditionally, external-frame packs are better at this, because the rigid frame allows gravity to carry the weight right down to the hips. I'm honestly not too sure what's available in NZ for those types of packs; in the U.S., the Kelty 50th Anniversary pack ($240; www.kelty.com) is a good one. You can also find some Dana Design Terraplanes and Shortbedsgreat packs that are no longer madeat closeouts.
Of course, you're also going to want to do everything you can to reduce the load. That may mean buying some lighter gear, such as lighter rainwear and insulation. It will also mean being very disciplined at leaving things out. And, maybe you can find someone to carry a few extra pounds for you!
Good luck with all this. Hope the surgery is a smashing success and you are soon back to your tramping ways.