well,..
The already purchased tickets will be valid for the duration of your employment(as of the start of notice and/or the validity of the ticket. That is also 3 months so you will be able to avail some additional tickets 1 day before you resign).
You do have a limit now for the numbers of tickets you can buy, opposed to before your resignation.(cant remember exact number right now 6 sets or so).
If the tickets are purchased before resignation (for your family member visiting and or planning to visit) there will be no consequences for them unless the tickets validity will expire. Again mind you this coincides with the 3 months.
Your week of leave now can be affected (withdrawn at will). Disregard a previous ACN(2012/2013) indicating that you would be allowed to utilize instead of payment of non used leave.
But don't be shy and just ask the separation officers in advance. Nobody will be able to stop you anyway.