Criminal law, traffic matters, DUI, assault, theft, fraud
Unlikely that they will be enforced outside Australia, but if you ever return at any time they will, possibly with other fees for late payment.
It is also likely that the authorities in your own country will be notified of this speeding and your license record updated to include the details.
Note that speeding is not a minor offence, the results can lead to the driver losing control of the vehicle and becoming involved in a crash, and that can include fatal consequences. As a student on limited means you should know that you need to abide by all laws, including speed limits or those limited means could be placed in jeopardy.
Of course it wont be enforced out of Australia, there is absolutely no chance that Australia will contact Croatia police and advise them that you were doing 105 in a 100. Its a scary looking letter aimed to make you pay up so they dont have to do anything.
If you committed a serious criminal offence, then yes of course.
Speeding is a minor offence at what you were alleged to have done with a 105 in a 100, in any other country it wouldnt be a fine, its criminal if you are intoxicated, un-insured etc.
I would just forget about it and next time you drive in Australia just drive 10% slower than Europe as slower here.
Further to citizen-joe's response there could be a complication with being allowed to drive here when you visit Australia next, although this will probably only be flagged if you get caught for another driving offence while you are here or you are subject to a licence check.
These days record keeping is pretty good. I'd hate to think of the complications if you returned here, had a car accident (even if it wasn't your fault), the outstanding fines resulted in you actually being unlicenced and thereby having no insurance. Probably worst case and not likely to happen.
Not sure if international licences are still used these days but you'd think some sort of infringement check would be made for the country that the licence was been applied for.
International Driving Permits are issued by auto clubs on the production of a valid (in this case) Croatian licence. Basically worthless except for some reason in Japan where you need one to rent a car or in Bali to get out of paying a bribe (fine) for not having one if stopped by a copper. Rental car companies don't (yet) have access to your licence details so it won't be an issue renting a car anywhere. It also won't affect your insurance as claimed by Eddy because you'll be using a Croatian licence and the Australian govt has no jurisdiction over Croatia.
I got a fine from NZ in the mail (to Australia) and I paid it - mainly to avoid hassles in the future if I travel to NZ and get pulled over by an actual NZ police officer who does an actual licence check or in case I decide to migrate there in the future.