Read
this link explaining how to enrol at Monash without a TFN.
In short, you will need to indicate that you will pay upfront first. Then when you receive your TFN, update it on WES when you change it to deferred. You must ensure that you provide them your TFN before the due date and census date. This will allow you to at least select your units and accept your offer.
To prevent your account being blocked or your position at the uni cancelled (should there be delays in receiving your TFN to enter in before the dates), you can follow
this link explaining how to provide Monash proof that you have applied for a TFN.
I recommend trying to get your TFN application ready to go so when you arrive back, you can print it and immediately get the Australia Post receipt and Application Summary documents to give to Monash. Bear in mind, the form is only valid for 30 days (i.e. you must visit a participating Australia Post within 30 days of completing the form), so try and match the timing on when you complete the form online.