What a terrible situation to be in @KezzaG. Nobody should have to go through all this stress on top of what they are dealing with from their illness and treatment. Yes, it seems to be the case that you have to be desperate before "they" will release what is in fact your own money. Who makes these decisions? I think there are three ways to deal with this, one to go very public about it, one to keep your job but stop your income so they have to give you compassionate access, and the other to find a private person, family or friend, to help out.
1. You could go back to the ATO and appeal to someone higher up than whoever you have been speaking to on the phone - and it has to be in writing. Nothing anyone says on the phone has the weight of a written statement. Send an email or letter with brief details. All you are asking for is compassionate release of funds approval for a limited amount like two or three months income equivalent. Send the same email/letter to someone high up in your Super fund AND with a copy to your local member of Parliament and one to the Minister for Health as well. You can get names/addresses from online records. If you get the brush-off, or no response at all, call one of the media outlets, Channel 7 or 9. This is a very good time for querying the way our institutions DON'T serve us well.
2. If that doesn't appeal, you could ask your employer to give you a few months leave without pay. Then, with no income, but still with a job, you could tell the bank this and ask them to send a letter stating that you are now in mortgage foreclosure. This does not look good on your credit record though.
3. To avoid all this, maybe you might have a family member or friend who could loan you the money you need to keep going without all this hassle? If you are working you will be able to pay them back eventually. A private arrangement keeps you out of the bureaucratic nightmare. Almost everyone on this network seems to have been able to go back to work in spite of what they have gone through. Their resilience and determination is amazing! It sounds as though you will be the same. Good on you. Stick up for yourself, you will get there eventually. Good luck and let us know how things turn out.