Once, when we kinda bypassed the doctor (for speed) we found that things like blood tests, scans etc attracted a gap payment although the majority was covered by Medicare.
One doctor, who had come to the end of any help he could offer, suggested we phone around to find a specialist that could see us quickly. He gave us two letters of referral - one to the doctor he suggested and one untitled so we could choose our own. This was a far cheaper option than going without a letter.
So, I suppose our experience has been that we haven't had to pay for anything for tests when it's been an emergency or when our doctor has given /sent a letter of referral.
I don't have a definitive answer though, just that it seems a lot cheaper if any work is ordered by someone we have been sent to via the 'ordinary' route. That said, it can take a tad longer so, as with most things, it's down to the consumer.
Good luck and best wishes to your OH.