You better get used to the PR way of dealing with the burocrats: take a day off and go to the place. Trying to get your info by calling, emailing or sending a fax is useless in 99% of the cases.
At the DTOP (Departamento de Transportación y Obras Públicas) you may find somebody who speaks some English but I recommend to take a Puerto Rican friend to make things easier.
There are two possibilities:
1. A treaty between PR and Australia exists and you will be able to exchange your Australian license for a PR license.
2. There's no treaty and you'll have to take the test to get a PR license.
If you plan to stay for a limited time I'd keep the Australian license and take the test for a PR license.
It's no big deal; first you have to take a theoretical test. This can be done in English and in Spanish. Buy the book with the rules (available in English) in Wallgreens or another drugstore.
After you pass the test you will have to take a driving test (two times 'round the block) and you're all set. You need to bring a car and somebody with a PR license who drives the car.
Your non-PR license is good for 120 days after your arrival, after that you need to have a PR license.
Ah, before you go and take the test you need an eye test and a basic medical exam and a couple of photos. Close to every DTOP there are businesses offering these services for a couple of bucks.
Now, before you can get a license (exchange or a new one) you need to have a social security card. For almost everything more or less official you'll be asked for your SS number. From getting a driver's license to opening a bank account.
To get a social security number you'll need a permit of stay - either a visa or a green card (permanent resident card).
The complete procedure for getting a SS number is here: http://www.ssa.gov/ssnumber/
(this site is in English)
There's a SS field office in Mayaguez:
Mayaguez Mall Shopping Center
State Road #2, Route 343
Mayaguez, PR 00708
Back to the DTOP, in Mayagüez they are here:
Bo. Sábalos Carr. Núm. 2, Km 156.2, Intersección PR-114
PO Box 1380
Mayagüez, PR 00681
Teléfono 787- 834-1515; Fax 787 834-2666
[edit]When I changed my license they would only give me a car license, although I had a German license for motorbikes and trukcs up to 7.5 tons. [/edit]