So I don’t know about Spanish criminal law in particular, but in Germany the criminal code states that it’s generally only applicable for acts committed on German territory, but there are multiple exemptions for offences committed abroad „with specific domestic connection“. This includes so called „offences against sexual self-determination“ if the offender is a German national. I could imagine that Spain has a similar rule.

Update: Seems I was (mostly) right, but spanish law is even broader than that of germany in these cases.

The Spanish courts shall also have jurisdiction over acts recognized as offences under Spanish criminal law, even if they have been committed outside the national territory, provided that the perpetrators are Spanish nationals or foreigners who have acquired Spanish nationality after the commission of the crime, and subject to the following conditions: (a) The act is an offence in the place where it was committed; (b) The victim or the Public Prosecution Service has filed a complaint or brought a case before the Spanish courts; (c) The alleged perpetrator has not been acquitted, pardoned or convicted abroad, or, in the latter case, has not served his or her sentence. If he or she has served part of the sentence, this will be taken into account in order to reduce proportionally the sentence imposed.

https://www.un.org/en/ga/sixth/71/universal_jurisdiction/spain_e.pdf

If it's illegal in Australia and Australia decides to prosecute, then you use Australian law in front of an Australian Court.

Here the harmed party and the alleged offender are both Spanish citizens. If there's a Spanish law and 'Spain' decides to prosecute, they'll use Spanish law in front of a Spanish Court. A Spanish Criminal Court can't sentence someone on the basis of Australian Criminal law.

Not sure if relevant, but in recent years, several countries have proclaimed universal jurisdiction for certain crimes. Increasingly often, countries assert the right to prosecute their own citizens for abuse of children even when committed abroad, which has been deemed necessary to counter "sex tourism".

Given that this is an accusation of sexual assault, I won't be surprised if Spain claims jurisdiction for sexual abuse of/by its citizens anywhere in the world.