Take a look at the map in the original article. The areas covered in either blue slashes or blue hatches (i.e. left and right slashes intersecting), which is most of the US, has "Don" = "Dawn". There's also the "on" line, so for those who distinguish "Don" and "Dawn", some Americans pronounce "on" as if it was "awn".

For me "tonic" has a fairly short vowel in the first syllable. Whereas "Tawnic" would have a longer vowel. There's also a different in sound. You might hear my "aw" as if it was a long "o" sound. The Australian vowel system is really nothing like the American vowel system.