I have a set of OEM 18" rims (but no roadworthy tyres unfortunately).
You might be better trying to find someone locally (put a post on
Zclub - Australia's Largest Nissan 370Z and Nissan 350Z Forum which is a local forum and you might get a faster response).
Nissan material for MYY2009 M6 and AT7 transmission quotes front track at 1550 and rear at 1595 .... fitted with either 8 x 18 ET43 front and 9 x 18 ET15 rear OR 9 x 19 ET47 front and 10 x 19 ET30 rear.
Are you sure you are being hassled over rear track because saying you need 1550 implies the front track is the query.
You can find all you need to know here:
http://www.infrastructure.gov.au/roa..._2011%20v3.pdf
Look on page 23 under section 4.2.9. Basically your seller has screwed you as there is now way a 20 x 8.5 is legal given the track figures you are quoting and if you did not purchase with a roadworthy certificate, it is up to you to fix. If you did have a RW certificate, then you can hassle the shop that issued the certificate becasue they have certified a car that is not correct and for that, they can lose their RWC license - you could try them to see if they can scare-up a legal set of wheels to get the vehicle registered.
The reg's say you can increase the track by up to 25mm overall BUT cannot decrease the track. Your previous owner has reduced the track based on the numbers you are quoting.
Without knowing what the 20" wheel offset is, it is a bit difficult to figure out, but a 10" wide wheel at +40 is not going to cut the mustard, for 10" wide, you need a maximum offset of +30. andcan come down as far as 18 and still be legal (might not fit under the guards at +18 though .. you'd need to do your research.
You can PM me on this forum or the zclub.com.au (same nickname) if you want more details.
Rgds
RB