18 inch on EA-EF aren't illegal but spaces are
Pre Au have large boss on the rim AU/BAs don't
so get the right rims from a reputable tyre dealer
You can step up in rim size so long as you follow your states rulings
QLD for example states that the over diameter of the wheel can be -15mm
to +23mm of the orginal diameter (this doesn't throw out the speedo to much).
so you can step from a 205/65R15 to a 225/50R16 to a 235/45R17
to a 235/40R18 and stay in those legal diamenions

just for noting the first 3 number are the width if the tyre in mm
the next two numbers are the perecentage of the of the witdth which give you the height of the sidewall (ie 40% of 235mm for the 18)
and 18 is the diameter of the rim in inches

all things being equal all you really have to be concerned with is the
load and speed rating of the tyre and thats more a necessity with low profile on tyres on utes/commericals
but thats a different storey for another post.

Biggest factory wheel on the EA was 15", 18" on an EA would be illegal.

The rim diameter can only increase by two inches in height, and one inch in width i think