G'day Bushed
Here in Australia, the term Mountain Ash generally refers to the Eucalypt species
Eucalyptus regnans
I can't say I have used Mountain Ash (more common on the border areas of NSW & Victoria, extending Sth along the Great dividing range and also found in our Island state of Tasmania).
As a matter of fact, it was the Mtn Ash forests that saw the brunt of the bushfires in Vic early last year.
They are trully magnificent trees.
Kind regards
Mick