Depends on when you moved your tax home, but you simply check the box on the front of the T1 and tell them you're no longer resident. You can get hold of the non-resident dept. in Ottawa and tell them you're non-resident also.
Bear in mind you have to cut all residential ties to Canada (healthcare, driver's licence, etc.) to be able to do this, Form NR-73 lists them all. You can keep a bank account in Canada but it's very important you tell the bank that you are no longer resident for tax purposes so that they withhold Part XIII non-resident tax from any interest they pay you.
If you can't move your tax home to
Austria then you'll have to claim a foreign tax credit on your T1 against any income tax you've paid in Austria to avoid dual taxation.
You can write them a letter of explanation to say you moved your tax home on such and such a date, but at this point it's probably going to be simpler to say you cut all residential ties on December 31st of the year that you moved, otherwise they will cut your personal exemptions and you will still owe tax and penalties on those days anyway.
Steve.