Oz medical *or* an FAA medical is acceptable. The 61.75 certificate is valid based on the validity of the foreign licence. A 61.75 cert is limited to any limitations the foreign licence has, even if an FAA PPL would not normally be so limited. If the foreign licence has greater than FAA PPL privileges then the FAA PPL limitations are controlling.
A flight review within the last two years is required for any certificate to be used. A flight review has to be with an authorised instructor - and the only instructors authorised to conduct those reviews are FAA certificated instructors. End result is that a foreign flight review doesn't count, even if it was done yesterday.
You will receive a letter that will state you need to go to a FSDO to have the 61.75 issued. You will usually need to give at least 24 hours notice for an appointment. After the cert. is issued find an instructor to give the flight review. Minimum 1hr of ground work, 1 hour flight. Bring your log book so the instructor can sign off the review.
Compared to Oz, flying here is very easy *but* procedures can be rather different. Buy a copy of the FAR/AIM & study it. You can also download the equivalent for free from the FAA's website.