We called a professional possum remover to get rid of ours.

I would be getting professionals in. Sounds like the problem has gotten too big for you to do it yourself

Humane Possum traps are provided by councils in our area

Do you know whether your Mum had to pay for the traps that were provided by Council or wildlife? All advice is very much appreciated thanks.

you need to call your local wildlife group, but in the meantime if you can try and find out how they are gutting in and seal it off.

My mother had the same problem. I think you can ring the council or the wildlife people in your area and they will give you traps to catch them and they will relocate them elsewhere.