Although you don't mention them, I am assuming you have your visas for Austria. But this is the info from the Austrian embassy:

Entry Requirements, Visa and Immigration

www.bmeia.gv.at

 

Hi, yes - visas are sorted, thank you. It's just about the forms required for transit of personal goods that we cannot get clarity on. Alex

 

Generally speaking, if you go through a customs agent, you need your residence permit (or visa, or other evidence of having permission to reside in the EU), port, a fairly detailed listing of what exactly you're bringing (inventory list) - including approximate date of purchase/acquiring, estimated current market value and (for things acquired within the last 6 to 12 months) purchase price (with copies of invoices if you have them). Low value items (say, clothing, kitchen utensils, stationery supplies, etc.) can be grouped with an estimated value and acquire date of "various."

In essence, you're functioning as your own "customs agent" so you want to have roughly the same sorts of documentation available.