AIUI, this is only if acting beyond the scope of any training received, or not otherwise acting in good faith. Ie: I only have a Senior First Aid certificate, so I'd be comfortable doing CPR. If, while doing so, I broke old mates ribs, I would still be held harmless. On the other hand, if I attempted to do a tracheostomy on some bloke choking in a restaurant and nicked his artery, I am a goner. You covered so long as you don't go above and beyond what a reasonable person, with your training and experience, would be expected to do in the circumstances.
By the same token, if you fail to act even though you have some training/experience in first aid or rescue, you can't be held accountable for someone subsequently dying, with the exception of in the NT - the only state to have such legislation, IIRC.