NSW Service Shop Damaged Computer - Legal Under Australian Consumer Law?

I dropped my computer off at a local repair shop. I asked them to clean out any dust, replace some worn/broken plastic pieces, and put in more memory. The shop sold me a new charger that was on special. I was given a written estimate of one month for the service and upgrade. When I gave them the notebook everything worked. When they returned it to me, I noticed there was an intermittent problem with it staying charged. A week later I returned it to the repair shop.

They tested it and decided it wasn't the new charger. They said it had a "short" somewhere on the motherboard. The owner insisted I had to pay for the motherboard to be replaced and wants to be paid again now that the replacement part she ordered was defective. All she told me is the motheboard she ordered is "refurbished" but I think she may have bought it from a dodgy ebay seller. It's been five months from when I gave them my notebook.

Is this legal under Australian consumer law? I gave a repair shop a notebook that worked. They gave it back not working. They charged me to fix it for not working, and want to charge me a third time because the replacement parts I paid for also don't work.

I would like to be told how consumer law applies in this situation. I need to know what the law is so I can talk her around to a compromise. I don't want to ask for too little or say I'm entitled to something that isn't right.

They never installed a replacement motherboard. The replacement was DOA. They want me to pay for the DOA board, plus pay for a second replacement board. The DOA board was apparent sold "as is" although they didn't tell me this. Since they can't get my money back they want me to pay again.