What consequences may occur as a result of driving a rental car in Austria without an International Drivers Permit?

Your Australian driver's licence is only valid in Austria together with an International Driver's Permit or with a translation issued by an authorized Austrian organization, e.g. ÖAMTC. If you are not able to get an IDP in Australia before you leave, it should be quite easy to get a translation from any of the ÖAMTC offices in Austria. They charge €14 for this service.

Driving without an IDP or a translation will be deemed as driving without a valid license. It is questionable wether the rental company will give you a car at all and if you are caught, the pentalties start with a 363€ fine for a first time offence. Repeating violations may bring you in jail. If you cause an accident, you must also expect the insurance to be void and you will have to cover own and third party damages yourself.

An International Driving Permit is required by law for Australians in Austria, it does not depend on rental companies' policies in this case.

Source 1: Austrian National Tourist Office

An international drivers license is required and can easily be obtained from your local auto club. The major U.S. rental companies are represented in Austria as are a range of European firms. Many credit cards offer free supplemental rental car insurance, which might let you save on optional car insurance. Check with your credit card company.

Source 2: DriveAway Holidays, an Australian self-drive company working in Austria

Source 3: Austrian Embassy in Washington (there was no information on the Canberra embassy's site)

According to Hertz car rental in Vienna airport:

For all Non-European Union renters an International Driving Permit (IDP) is highly recommended. For renters from the US and Canada it is required.

According to Avis Vienna airport:

Additionally, all driving licenses printed in a non-Roman alphabet must be accompanied by a valid International Driving Permit (IDP). Both forms of documentation must be issued to the same person, in the same name, and presented together at the time of rental.

Likewise according to Sixt Vienna airport:

Driver's licenses from non-EU countries (except Switzerland) are accepted if:

• no visa has been entered in the passport.
• the customer has a visa in his passport and at the time of hire has not been in Europe for longer than 6 months. If he has been in Europe for longer than 6 months, he must present a driver's license from an EU country.

Driver's license printed with non Roman Alphabet (Arabic, Chinese, Japanese, Cyrillic etc) must be complemented by an international driver's license.

For driver's licenses from countries not part of the international driver's license treaty, an official translation of the license must be presented with the original license.

Compare this to Hertz in Narita airport:

Japanese law requires that all drivers who do not hold a Japanese driver’s license must hold a country of origin driver's license and an international driving permit issued under the 1949 convention on international road traffic. No other International Driver Permit will be accepted for rentals in Japan.

Or to Sixt Australia:

The renter and all approved drivers must be in possession of a full, valid driving licence for at least one year. Furthermore, the following conditions apply depending on the issuing country of the driving licence.

International renters/drivers also need an international driver's license and a valid passport.

This leads me to believe that in practice, an IDP is not required to rent and drive a car in Austria with an Australian license. If it was, all the rental companies would insist on it in their T&Cs.

Quote from the Austrian Führerscheingesetzt (Law for divers licences).
For the original text in german, see below.
_{The translations are mine, I am not a lawyer nor a legal translator of any sort.}

Foreign drivers licences § 23

Sections 1 to 4 only concern persons with residence in Austria and are therefore not relevant.

Section 5 states that the person must at least 18 years old. And must not stay more than 12 months in austria.

(6) As proof of a permit for driving the according national drivers licence must be present. If it is not (also) written in german and does not conform to the exemplary from attachment 9 of the geneva convention or the contents of attachments 1 or 1a of the policy 91/439/EWG, ABl. Nr. 237 from August 24. 1991, version 97/26/EWG, and does not conform to the requirements of attachment 6 of the Vienna convention, the drivers licence must be accompanied by an international dirvers licence based on a convention mentioned in section 5 or with a translation by an authorized association (based on § 36 section 2 Z 3) or a foreign representation of the issuing country.

I tried to keep the translation as verbatim as possible. Any improvment are welcome.

Conclution: You need to check which, if any, conventions the Australian divers licences follow. I recommend you check with one of your local car associations. They usually know the relevant regulation for travellers and can also point driving rules that are different from your home country you have to watch out for.

Orignal text in german

Ausländische Lenkberechtigungen § 23

Absatz 1 bis 4 betreffen Personen mit Wohnsitz in Österreich und sind hier nicht relevaten.

Absatz 5 besagt, dass die Person mindestens 18 Jahre alt sein muss, und sich nicht länger als 12 Monate in Österreich aufhalten darf.

(6) Als Nachweis für die Lenkberechtigung muss der entsprechende nationale Führerschein vorliegen. Wenn dieser nicht auch in deutscher Sprache abgefasst ist und auch nicht dem Muster des Anhangs 9 zum Genfer Abkommen oder den Inhalten des Anhangs 1 oder 1a der Richtlinie 91/439/EWG, ABl. Nr. 237 vom 24. August 1991 in der Fassung 97/26/EWG, entspricht und auch nicht die Anforderungen des Anhangs 6 zum Wiener Übereinkommen erfüllt, muss der Führerschein zugleich mit einem internationalen Führerschein nach einer der in Abs. 5 angeführten Vereinbarungen oder mit einer von einem gemäß § 36 Abs. 2 Z 3 ermächtigten Verein oder einer ausländischen Vertretungsbehörde des Ausstellungsstaates verfassten Übersetzung vorgewiesen werden können.