Well, only if you enter by air or sea. You do not need ESTA authorization to enter by land.
Furthermore, persons traveling to the U.S.A. under the visa-waiver program may only stay for 90 days and cannot extend this time. The six month time limit is for persons entering Canada only.
Since you were an Australian passport-holder you do not need a visa to travel to Canada or the U.S. under the Visa Waiver Program. However, you do need to apply online, free, an ESTA (Electronic System for Travel Authorization) at least three days before you wish to enter the U.S. You may stay as a traveller in either country for as long as six months unless the Custom or Immigration Officer at the border put a limtation on your passport.