Wow, we are a spoilt lot. We have so much riding and we talk it down.
Lots of good info from many here but I'll sing the Melbourne praises. Within 10 mins of the CBD you can be on the Yarra Trails, which is xc. If you know how to link up all the trails you can do a handy 84km loop starting in Richmond near the breweries head out twd the Mullum and beyond then back on the other side, hell you can even throw in the tunnel under the freeway if you haven't done that and the weather permits, not saying I have done it and not saying I have not, just saying it is there, not too legal tho.
Also within 10 mins of the CBD you have Westgate Park. If you are strong yo can link up the Yarra Trails with the Dandenongs. Otherwise a 45 min drive from the CBD has you riding for hrs in the Dandenongs.
50 minutes by car has you riding in Macedon and Woodend / Wombat, same time to SG or Kinglake, 60 minutes to the You Yangs, 75 minutes to Castlemaine, 90 minutes to Creswick, 1hr 45mins to Lake Mountain / Buxton trails, Redesdale, Anglesea, Mount Buller, Stirling, the magic of Mount Beauty, Beaconsfield, Whittlesea / Mount Disapointment, don't forget Forrest, there are trails around Sale and further east, also thre is Lysterfield about 45 mins out of town. This is not an exhaustive list.
Yes some of these are further out, but this state has heaps and all is easily accessible.
I love the stuff in Canberra but you will freeze your cock off in Winter, QLD is great but spread out, Radelaide is rad but smaller, I can't comment personally on the other places, I reckon Tassy would be pretty awesome though having been there for work a few times, but if you are a xc rider Melbourne is the place to base yourself and will have more work potential I believe.
And yes there are a lot of hills if you want them.
For cross training it is also a great place for road riding (sorry for blaspheming here) but if you head north east you will not be disappointed, just stay away from Beach Road heading south, boring as bat 5hit, mmmm north east or east has great hills.
Regards,
Mars.