Barn Owls and Brolga's

Today, i went down to Reedy Lake, Leopold, Victoria (Moolap
Station Rd) for lunch. 

I was standing on the mound at the end of Moolap Station
Rd, when i saw a large white bird leap from the bushes, being harrassed
by some magpie larks. 

I put my scope on it and it was a Barn Owl! (i think).
It flew across the road and towards the swamp, where it swooped three Brolga's
grazing in a burnt out area next to the swamp.  The Brolga's ducked
their heads in cover, and the Barn Owl flew up and landed in a tree on
the edge of the swamp. 

Is anyone able to tell me what the likelihood would be,
of it being a masked or grass owl? What features distinguish the three
species?

Peter