I've been told that once you have registered with the Polis your details are automatically sent to the tax people and not only will they tax you on any future stays but they can also charge you backdated tax for up to 5 years. .
This is pretty much correct. The Tax Office is already checking that they have a matching Tax Return for every Police Registration.
If you have not registered for Tax they will issue an Assessment from the date of Police Registration. This process has already begun and assessments have been sent.
They are also sending Assessments to any owners that were issued with fines for non-registration during the course of last year.
Like any Tax Authority in the world they do have the power to investigate you for previous years (In England it's seven years).
However, if you have registered and paid all your taxes during the course of last year they have unofficially sort of declared an amnesty on previous years activities.
If you decide to argue with any assesments, fines, penalties etc they have the five year rule to invoke and it's a big stick to beat you with.