Does LibreOffice already use Macquarie for Australian English?
I sincerely doubt it. The Macquarie dictionary is likely copyrighted by Macmillan Publishers Australia.
Does it offer UK English or Australian English?
Yes. Both. This is what my MacOS install shows:
$ ls /Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_*
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_AU.aff
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_AU.dic
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_CA.aff
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_CA.dic
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_GB.aff
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_GB.dic
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_US.aff
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_US.dic
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_ZA.aff
/Applications/LibreOffice.app/Contents/share/extensions/dict-en/en_ZA.dic
The dictionaries are in ASCII format but have a specific layout requirement. The initial entries in the en_AU.dic file are:
49845
1st
2
2nd
2th
3rd
7
7th
A
AA
AAA
Aachen/M
Ababa/M
abalone/SM
abase/LDSG
abaser/M
abash/LGDhS
abbreviate/nDGNS
Aberdeenshire/M