If you are a new researcher (PhD students, Post-Doctors, and Professors) coming to Austria, you may be entitled to the immigration tax benefits, which reduces your tax rates by 30%.

Please check the URL: https://www.euraxess.at/austria/information-assistance/taxation-and-salaries

More information can be found by searching "Zuzugsbegünstigungsverordnung".

Perhaps, the Ph.D. students can be also qualified for this tax policy because the check-list of new Ph.D. students in TU Graz mentioned this regulation.
https://www.tugraz.at/fileadmin/user_upload/tugrazInternal/Studium/International_studieren_und_lehren/Welcome_Center/Checklist_international_PhD_student_and_staff_English.pdf

"Check whether you are entitled to a favourable tax treatment for highly qualified researchers (Zuzugsbegünstigungsverordnung) by the Austrian Federal Ministry of Finance. For further information and advice, please consult a professional tax advisor."

Important: please note that you can only apply for this benefit within the first 6 months after moving into Austria.

Please also consult other colleagues, who have applied for it successfully before.   

Good luck with your application.