The prosecutors’ office in Madrid said that, according to a sexual consent law passed last year, Rubiales could face a fine or a prison sentence of one to four years if found guilty. The new law eliminated the difference between “sexual harassment” and “sexual assault,” sanctioning any unconsented sexual act.
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.