Does anyone know if the 400,000 baht deposit needed for a 1 year visa renewal have to stay in the account for the full period while the visa is being processed ? [usually 2-3months]

Australian passport.

It is subject to check at any time during the review process so best to keep it in the account or, if required for emergency or such, let immigration know so they have a reason on file. Some people have reported long delays in approval being caused by immigration waiting for account to come back up. I suspect these cases are few but best to not take a chance; especially if a large amount.