how to track parked domain with google analytics?

Short answer: Yes you can, sort of

Rationale: in fact, only you know it is a parked secondary site, not "a" main site. The only problem is to track visitors and how they came here.
The trick is that, most probably, you have done a htaccess redirect to your main site, and so there is in fact no web page in which you could put the Analytics code....

How I would proceed:
1 - Looking at your (present) main site analytics, you will find amongst your "referrers" the old site: this will show you all the traffic that was redirected from old to main BUT you will probably loose the additional information (where did traffic come in the first place, which pages were linked to, etc.
2 - Even if you are not using Analytics, you probably have some information on traffic incoming to old site: eg, Apache logs. You might even already have some interface that makes easier to explore these logs; probably not as easy as Analytics... but powerful enough to help you find:
  a - which pages are linked to
  b - which are the main referrers (including search engines, in which case you also know with which keywords)
  c - who, if any, are the main visitors to your old site
3 - From there, you will find if you need any additional work or if the present situation is "good enough". The 2 extremes might be a lot of traffic with a high-ranking keyword (and it would be a pity to ignore and lose that!) or an almost non-existent traffic.
If you get lots of traffic, then you might consider creating a small site to handle it effectively, with the 4-10 pages most linked-to created to handle effectively a 301 redirect.
4 - And to be sure of what remains in the search engines indexes, look in GYM (Google, Yahoo, Microsoft) after pages that contain the string " site2.com.au": these are pages in their indexes that point to your old site, you may address each of the hosting webmasters to give them the info.

REMARK: since you are building your business around site.com.au, you MUST look at what is happening on "site.com". If the domain is available, BUY IT NOW. If it is not, check if its owner is not reaping some rewards from all the marketing YOU are doing

Just an addition to 3 above
If you create ad-hoc pages, then of course you can add analytics page to the corresponding pages.
You have 2 strategies here:
- use the same code as the main site, so that you have a "unified view" of the traffic
- my recommendation would be more to use a specific analytics code, and certainly specific reports