I can confirm that if Tantawan wishes to withdraw from her packaged offer due to not meeting the English entry requirements she can do so by notifying both MIT and MU that she has no intention to continue on with her studies.
If this is what Tantawan wishes to do then, Murdoch University will report on the students COE (proposed start date as 2014) as non-commencement due to the student not meeting entry requirements and advisement that she does not wish to continue with offer. Once this has been confirmed, we would recommend that Tantawan contacted the Department of Immigration and Border Protection to discuss her visa.
If the above is the way Tantawan wishes to proceed then I can confirm that a release letter is not required.
Please let me know if you would like us to proceed with the cancellation of her COE due to non commencement.