The sentence does seem harsh to me too, but generally speaking ignorance is not an acceptable defence espscially when the ignorance is not knowing/caring about the scale of the offence rather than just being ignorant that it is an offence at all.
Quite possibly. Intent often matters, e.g. involuntary manslaughter vs murder.
As for murder, the penalties there do differ depending on your state of mind, but that's not an issue of mistake (you can't try to third-degree murder someone but mistakenly first-degree murder them).
Not to say I agree with the punishment in this case, though.
In the theft and criminal damages context, you could have an analog, though we don't currently, where being judged guilty of one of the more major theft or damages offenses (like "theft over $X") requires an intent to cause that level of theft or damages. If you meant to cause $50 in damages and actually caused $50k, that could be a lesser offense than if you meant to cause $50k in damages. You'd be found guilty of essentially "damage over $X but with intent to only cause damage under $X", the way 3rd-degree murder and manslaughter are lesser offenses due to the lack of intent to cause death, even when there was intent to violently injure non-fatally.
When you get excused is if you didn't act purposefully... e.g. if you drive your car recklessly and kill someone you'll be guilty of manslaughter. If you purposefully hit someone with your car you'll be guilty of murder even if you only meant to break a few bones.
Basically they have a pretty tenuous, very expensive 3G network to serve an enormous sparsely populated area and they really don't want people to use it to download large amounts of data. They're happy to sell you a wired link for that purpose.
The problem here is Telstra and/or the power company for not securing their SIM card or account.
At the very least this "Mr. Freeman" should be involved in the case as he removed the SIM card, which is probably a violation in and of itself.