It seems as though it should be, but sometimes these things can surprise you. There may be a way the family members can perpetuate a copyright beyond what you might expect...
(I couldn't find anything conclusive on the web)
If I wanted to use it as the jingle in a commercial, could I just go ahead and use it like it was Beethoven's fifth or Brahms' lullaby?
D50
How could the Aussies have been so stupid as to reject Waltzing Matilda as the national anthem? It makes them seem like just a bunch of wowsers.
Robert J
Don't forget there are two copyrights involved:
The score / written music is a few years out of copyright, as Tony B says.
But, each and every performance of it is separately copyright by the performer(s).
You must "perform" it yourself, not use anyone else's performance.
Then you own the copyright to your performance of it.
It does not matter how you perform it, whether instruments or transcribe it in to a MIDI editor and play it on a computer; or pay someone else to perform it on your behalf.
Tony B
The song is credited to Banjo Patterson who died in 1941. In Australia copyright extends to 70 years after the death of the composer. That would mean the copyright expired in 2011. Therefore it's in the public domain. There's nothing family members can do to extend the copyright.
Be aware though that this applies ONLY to the song, NOT to any arrangement or recording of it.