The variation changes across the country. Go to
this GA page, fill in your location and get the exact figures. There are links towards the bottom of the page to JPEGs for the whole country to get an overall view.
Using the place name search for North Mclean Qld gives
this page.
Feeding those coords back into the page gets:
Australian Geomagnetic Reference Field Computation
Requested: Latitude
-27o 45' 00", Longitude
152o 59' 00", Elevation
0 km, Date
2011/01/1
Calculated: Latitude
-27.7500o, Longitude
+152.9833o, Elevation
0.00 km, Epoch
2011.0000
Magnetic Field Components
D = 11.059 deg
dD = -0.015 deg/yr
So your local variation is about 11 deg E, reducing by 0.015 deg/year.
As for how accurate you need to be, it all depends on what you are trying to achieve.
An equatorial mount has to be reasonably aligned or it will drive you batty.
An alt/az mainly needs the base to be level. The orientation can be set by pointing the scope at a known object and adjusting the setting circles so they match the target. Scopes like Celestron's and Meade's GPS models can work it out by asking you to point at a given star, you tell the program when the star is centered, and it does all the calculations by itself.
Mounts with smarts like GoTo or Argo Navis requires the guiding software and the scope to agree on where they are pointing. The better the scope is aligned, the better they work.
If you want to do photography, you have to be more accurate in your orientation. That would be a discussion for another day.