Hello Matthias,
I checked for the name Sellinat by using the Record Search link on the National Australian Archive website
http://www.naa.gov.au/Search/search.html
and no reference to the name was found. Since naturalisation records of that period would be stored at the archive, it suggests that Sellinat did not become an Australian citizen. Admittedly, there are many records that are not yet indexed.
Greetings from Brisbane
Eric Kopittke