Canberra has a bunch of idiots arguing in parliament so you probably wouldn't want to go there.
And lots of dives, unfortunately none of them are underwater...
Seriously though, Wingy's right. Distance will be the problem when it comes to the great dives in Australia. Your non-diving friends probably won't thank you for parking them in Exmouth (Ningaloo), Ayr (Yongala) or South West Rocks (Fish Rock Cave) because they're all small places that don't have much to do except dive. IMO Townsville and Brisbane are also borderline in this respect, although they're nice enough cities there are not a lot in the way of visitor attractions (much as their respective tourist boards would like to claim otherwise). You may face a mutiny.
Driving to any of these places apart from Brisbane takes an overnight trip or longer from the relevant capital city unless you're trying to stage an Australian version of the movie Vanishing Point (and he didn't have time for diving), so isn't feasible if you've only got a couple of days. If your friends are happy to stay in a small and rustic beachside place you could stay at Port Macquarie and commute up to South West Rocks, but again... a bit borderline IMO when it comes to non diving entertainment, and you'd have to drive from Sydney or Brisbane (about 550 km one way from either) unless you want to pay a
fortune for regional domestic airfares.
Perth is a very nice city to visit and has diving. Likewise Sydney and Melbourne, and for non-divers with only a few days in Australia these three cities are probably the most interesting/entertaining. That said, it depends on what they'd like to do. Adelaide may also be an option but I know absolutely nothing about it... no doubt someone else will have suggestions! Certainly it's regarded as a nice place and I think it has shark diving?
If they're happy to enjoy the beach, sit beside a nice pool and see a few local attractions you could go to Port Douglas or Cairns and dive while they do that. Likewise the Island resorts; Lady Elliott Island has a good reputation for diving, though I haven't been there myself. IME the day diving off Port Douglas is nicer than what's on offer off Cairns, but they're both pleasant days out and in any case the PD operators can pick you up in a bus from Cairns.
Byron Bay or the Sunshine Coast would be other beachy options. In Byron you could dive Julian Rocks (and maybe do a day trip down to Wooli) while your companions enjoy laid back gentrified hippy stuff like day spas, beaches and art galleries. Noosa (Sunshine Coast) is another beach resort where you could dive the HMAS Brisbane and your friends could relax at the beach and visit the national park. Both are popular with international tourists and have good diving. Byron is 2-3 hours drive from Brisbane Airport, and Noosa is about 1.5 hours from there and has limited domestic flights into Maroochydore Airport. Both areas have good restaurants, accomodation and are well set up for visitors.
Anyway, just my two cents. If you do a bit of googling you can see if the attractions in those places would suit you and your companions. Or you could sign them up for an OW Course and convert them to the cause!