Is your computer - Windows OS - set up correctly to use 'Timezone' for Australian Eastern Standard Time setting ? On your computer..... Settings > Time & Language > Date & Time Is 'Timezone' set up for UTC +10 Canberra, Melbourne, Sydney  ? What have you got set up ?

Regarding your version of Thunderbird... is it using 'English British' or 'English US' or something else - what are using?

In Thunderbird Settings > Calendar What settings are selected ? is this selected 'Use system timezone'? Or did you select 'Set timezone manually' and then select 'Australia Melbourne' ?

Thanks, more information: Windows OS Timezone: I have sleceted (UTC+10:00) Canberra, Melbourne, Sydney directly .. I halso have Adjust for DST automatically set to On (I was recently on holidays in Perth and the time zone didn't sort it self out) TB Language: English US is the setting under Language Settings> Calendar is set to 'Use System Timezone.

Thanks for ideas of where to look