This is how to change the Date Format in O365 online apps to the Australian Date Format (DD/MM/YYYY) instead of the US Date Format (MM/DD/YYYY).
The preferredLanguage Property needs to be changed for the user in question in AD.
This can be done manually by going into AD, going into the User Profile (make sure Advanced Features is on in the View menu) and change the preferredLanguage in Attribute Editor to the desired language (en-AU).
This can also be done in bulk in Powershell by running the below script:
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$cUsers = Get-ADUser -filter '-not preferredLanguage -eq "en-AU"' -searchbase "OU=.Cape York Partnership,DC=cyp,DC=local" -Properties preferredLanguage
$cUsers |%{
Set-ADObject -Identity $_.DistinguishedName -Replace @{preferredLanguage="en-AU"
}
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What this does is finds all users in the ".Cape York Partnership" OU and sets the preferredLanguage to en-AU.
There is a script set up that runs periodically to do this, however, just documenting what this does and why.