Jetabroad is one the few Australian based online providers that offer flights that depart from anywhere in the world, most restrict to Aust. point of orgin.

For return and one-way searches, you can search by alliance, OW, SA, ST. Allliance search not available for mulit-city search.
For BUD-KBP / KBP-VIE, you can use the one-way search as BUD-VIE, with KBP as a stopover.

http://www.jetabroad.com/