Former Governors-General are usually styled ‘The Most Honourable Sir’. They would not however, be styled as ‘His/Her/Your Excellency’. The formal Mode of Address is:
In writing:
The Most Honourable Professor Sir Kenneth Hall, ON, GCMG, OJ
Address
Opening Salutation:
‘Dear Sir/Madam (formal)
……………………………………………………..
Yours faithfully (formal)
Yours sincerely (semi-formal)
Signature’
His wife is referred to as ‘The Most Honourable Lady Hall’.
Widows of former Governors-General would be referred to in a similar manner.