Tusa is by far the best day dive operator out of the area. Top staff, boats and service. Yes, most definately suggest you take their guide service but let them know in advance so they can staff accordingly and have that extra divemaster on board. They may charge you but for your first ocean dives this is a good thing. From the US we get accustomed to diving in Mexico and other more local areas where all the dives are guided in groups-all you ever really see are fins in front of you! Australia does buddy diving only, so it's just two of you out exploring.
Also, Tasmania diving is spectacular- You need to contact Gary at Eaglehawk Dive just a bit out of Hobart - the kelp, wrecks etc are just fabulous- yes a bit colder than the reef but worth it and very good experience for a new diver-please let him know you are new / by the way all these people are currently in Las Vegas at the DEMA dive convetion, myself included, so a base reservation staff will be in the office for the next week.