The frustration is understandable my friend. But the information they told you is essentially correct from what I know. The logic boards are actually different so the authorized provider could not get replacement parts from an iPhone w/ different internal hardware (your Australian iPhone) from Apple even if they wanted to. It's not a question of them not wanting to be bothered, I'm sure they would love to charge you for a repair, but they can't.
And this won't help you a lot at this point, but maybe some: in a humid climate like Sri Lanka you would not want to attempt to power on or charge your iPhone for a good 72 hours to be sure that all the moisture evaporated. I can see though if it was only for a few seconds not doing that (but that would be a mistake). In someplace like Australia, any water that leaked in would evaporate much quicker (such as in a day or so). That being said, at this point I strongly suggest (especially since it's not working anyway!) that you set your iPhone aside for a few days and cross your fingers.
Hopefully some moisture is simply creating a short circuit and once it evaporates it will turn out that no permanent electrical damage has been done. However, every time you turn it on/charge it/or try to use it you increase the chance that an electrical short will do permanent damage. You may want to consider getting a temporary generic phone and moving your Australian SIM card into that in the mean time.
If you do happen to have AppleCare+ accidental damage protection, you will be able to use that once you go back home. Otherwise, it's generally better to just buy a new iPhone or to use insurance versus getting it repaired if it still won't turn on after a few days. If you don't have AppleCare+ damage protection, and don't have insurance through your mobile carrier, then with water damage, typically getting it serviced costs about 80% as much as a new iPhone of the same model, and only gives you a 90-day warranty.
