It's illegal to use the word "tire" in Australia!! We use the Queen's English here, therefore "tyre"
Anyway, Chris has pretty much nailed it. Once you have checked the applicable laws in your State for the maximum allowed increase, just compare the labelled size of the tyre you want vs your vehicle's tyre placard and see if it complies. In terms of compliance, you really don't need to worry about the actual size of the tyre, just that the labelled size as sold fits within your limits.
I assume that you have the 245/75R17 GY Wrangler Silent Armours, which are approx. 31.5" (i.e. 245 x 0.75 x 2 /25.4 = 14.47" + 17" = 31.47") or 800mm, whereas 285/70R17 are 32.71" or 831mm. So if you are allowed to go 50mm bigger, you could go to a 295/70R17, which is 33.25" or 845mm.
Hope this helps?