I wish I had a dollar for every time someone has asked just how big an engine they can jam into their FB/EK and still legally hang number plates on the front of it

A few caveats:
a) The info below is for guidance. I’m no lawyer, rego authority or policeman – use the info at your own risk. No promises that I haven't misread the websites or missed something (if I have please post and let us know).
b) In most cases, engineering signatories have some leeway to go over or under the level of control in this material. It is a guide only – don’t be surprised on one hand if your mate can get a 600ci motor engineered, or on the other hand if a particular engineer asks you for four wheel disk brakes with brake fail warning lights for a red motor.
c) The information changes regularly.
d) There is a heap of kerfuffle over the implementation of the National Code of Practice… don’t be surprised if your engineer will do nothing until the kerfuffle is resolved.
New South Wales
Home page
http://www.rta.nsw.gov.au/registration/ ... index.html
List of guidelines
http://www.rta.nsw.gov.au/registration/ ... infosheets
Specific guidelines for light vehicle modifications
http://www.rta.nsw.gov.au/registration/ ... v_2007.pdf
Queensland
Home page
http://www.tmr.qld.gov.au/Safety/Vehicl ... tions.aspx
Description of process
http://www.tmr.qld.gov.au/~/media/c792a ... icles2.pdf
Code of Practice
http://www.tmr.qld.gov.au/~/media/b3ea8 ... ctions.pdf
South Australia
SA's rego authorities are somewhat shy about publishing modification guidelines.
List of guidelines
http://www.transport.sa.gov.au/personal ... hicles.asp
Modifications:
http://www.calaisturbo.com.au/attachmen ... 1149671499
Victoria
List of Guidelines
http://www.vicroads.vic.gov.au/Home/Saf ... maiton.htm
Specific modification guideline
http://www.vicroads.vic.gov.au/NR/rdonl ... hicles.pdf
Vehicle Standards Bulletin 14 (VSB14) referenced by guidelines
http://www.infrastructure.gov.au/roads/ ... _ncop.aspx
Western Australia
Home page
http://www.transport.wa.gov.au/licensing/20500.asp
Vehicle Standards Bulletin 14 (VSB14) referenced by guidelines
http://www.infrastructure.gov.au/roads/ ... _ncop.aspx
Tasmania
Home page
http://www.transport.tas.gov.au/vehicle ... ifications
Vehicle Standards Bulletin 14 (VSB14) referenced by guidelines
http://www.infrastructure.gov.au/roads/ ... _ncop.aspx
Northern Territory
Homepage
http://www.nt.gov.au/transport/mvr/vehiclestandards/
List of Bulletins
http://www.nt.gov.au/transport/mvr/vehi ... ndex.shtml
Modification guidelines
http://www.nt.gov.au/transport/mvr/vehi ... /ibv32.pdf
Vehicle Standards Bulletin 14 (VSB14) referenced by guidelines
http://www.infrastructure.gov.au/roads/ ... _ncop.aspx
Australian Capital Territory
Homepage
http://www.rego.act.gov.au/registrations/regomain.htm
Modification guidelines
http://www.rego.act.gov.au/assets/PDFs/ ... %20ACT.pdf
Vehicle Standards Bulletin 14 (VSB14) referenced by guidelines
http://www.infrastructure.gov.au/roads/ ... _ncop.aspx
Looking specifically at engine capacity (the most common question):
NSW
naturally aspirated, in cubic inches is original tare mass (kg) x 0.294 and in millilitres (cc) is original tare mass (kg) x 4.82.
forced induction, in cubic inches is original tare mass (kg) x 0.244 and in millilitres (cc) is original tare mass (kg) x 4.0.
Queensland
Same formula as NSW, except Queensland lists the tare mass as 1099kg for FBs and 1120kg for EKs (I can't find an offical list of vehicle weights for NSW). This gives:
naturally aspirated: 323ci/5297cc for FBs and 329ci/5398cc for EKs
forced induction: 268ci/4396cc for FBs and 273ci/4480cc for EKs
South Australia
Transport SA does not specify an absolute maximum capacity, power and/or torque for engines that may be fitted to passenger vehicles, however, Category 1 and Category 2 Approved Engine Change Lists have been developed which specify some engine changes which are acceptable to Transport SA. I cannot get my hands on an online copy of the Lists. For other than Category 1 modifications a Statement of Requirements is required from Transport SA prior to commencing modification of a vehicle.
Victoria, Western Australia, Tasmania, Northern Territory and Australian Capital Territory
naturally aspirated, in millilitres (cc) is original mass (kg) x 5.0
forced induction, in milliliters is original mass (kg) x 3.0
Can't find lists of original vehicle weights in the VSB14, but it does give a pretty good description of how to determine one at http://www.infrastructure.gov.au/roads/ ... 011_v3.pdf
Hope this helps.
Cheers,
Harv.