Australia and the U.K. are “Five Eye” nations (US, Australia, Canada, New Zealand, United Kingdom). We have 70 plus year history of sharing intelligence at the SECRET and TOP SECRET levels with these countries. The universe of information the U.K. and Australia are privy to dwarfs what they are not.
I am trying to figure out what information re Afghanistan the intel community would not want then to know about. The only NOFORN (not releasable to foreign nations) I saw while deployed dealt with politically sensitive matters (where our policy and theirs diverged) not intel related matters.
We keep a lot secret from them solely because allied military officers would be duty bound to report it, and allied leaders do not want to know so they have plausible deniability in case US actions have domestic blowback.
One of our main points is that we spy on their citizens and they spy on us. Thus, we ‘share’ it.