transit visa

[jag]

need some help
i am currently in u.s on the visa wavier program from australia
and i am flying to mexico 1 week before my visa expires i planned to stay in mexico for longer than a week but my flight to peru is via miami do i need a transit visa to get back through america if i am not leaving the airport..? is this possible after my visa waiver has expired...? or can i apply for the visa waiver again without a problem....?
thanks for your help