Regarding bottom times...
On the Yongala, even experienced divers would be lucky to do a 60 minute dive on a single tank. The bottom of the wreck is at roughly 28m and the top is at 14m. It's not unusual to see newer divers doing safety stops after 30 mins and then getting back on the boat after 35 minutes or so. I'm pretty good on my air consumption, and even so, I'm usually heading to the mooring line to do my 10m and 5m safety stops after about 50 minutes.
The GBR day trip boats that operate out of Cairns will enforce a 60 minute dive time in my experience - which has more to do with having to move the boats around and organising snacks and lunch than anything else. But what they do is they let the more experienced divers in first so that you're coming back at roughly the same time as the less experienced divers who might not be so good on their air. I've come back to the boat after 65 minutes on a day trip and they didn't bat an eyelid because I was coming back at the same time as everyone else. It's just a timing thing so that they can keep to schedule, they won't read you the riot act if you go over, as long as you're not totally blowing it and making everyone else wait.
The GBR liveaboards are far more lenient, and with Spirit of Freedom and Spoilsport in particular, they pretty much operate an open dive deck "the pool is open, be back on board with 30 bar". Not unusual for me to do 80 minute dives on Spoilsport.