Its not quite the same in the top octave, some whistles are more temperamental than others
on a D whistle
top C# (ie no fingers on in the lower octave) should work when overblown (ie blow harder)
top D (which at the lowest note is all fingers on) D usually works in the middle octave with all fingers on - although some charts show lifting the top (finger closest to your mouth)finger off but the very top D quite often won't play unless you take the top finger off and leave the other 5 on.
you can get a few more notes beyond that but the fingering goes a bit weird....