I know with Macquarie that you can fill in the form without your TFN except you have to choose to pay upfront for the meantime. Then when you get your TFN you just tell them what your TFN is and then you are eligible for HECS.
"If you are required to lodge your eCaf before you have your TFN then submit your eCaf on eStudent. However you will need to select the upfront payment option on the eCaf. Please note you are not locked into paying upfront for your fees as long as you provide a TFN or a copy of the Certificate to the Student Enquiry Service by the census date at the latest. Please note you will not need to go back and change the payment option on your eCaf,
as long as you have supplied your TFN by the census date or by the end of the 21 day validation period (if you have provided the Certificate by the census date). *
Once you have received your TFN contact the Commonwealth Loan Schemes Officer on +61 2 9850 6188 or 7323, email
eCaf@mq.edu.au, or complete the slip available at the Student Enquiry Service counter, Level 1, Lincoln Building (C8A). Once the TFN is received you will be entitled to a HECS-HELP or FEE-HELP loan. "
eStudent eCAF FAQ
*I know for Macquarie that their census date is the 31st March.