Evilbrian is this personal experience of yours? The Citi website seems to indicate otherwise ie. they will apply payments to the purchases first, but you get no interest free days unless you pay the entire balance (including balance transfer) meaning you'll accrue daily interest on every purchase until you make payment. I guess you could pay every day after a transaction to limit the interest to one day, but to be safe I'll use my NAB Velocity Visa during the BT period as the Citi rep was very evasive when I asked this exact question.
How will my Balance Transfer affect my interest free days?
You will not receive the benefit or be eligible for interest free days on purchases unless you pay the Closing Balance in full every month (including any Balance Transfers).
What is paid first, my Balance Transfer or other transactions I make on my account?
If your credit contract was formed on or after 1 July 2012, we will apply your payments to parts of your credit card balance with the highest interest rate first. If your credit contract was formed before 1 July 2012 and you haven't asked us to change the order in which we apply payments on your account, we will apply your payments to the parts of your credit card balance with the lowest interest rate first.