"Australia was certified free of malaria by WHO in 1981, but it is believed that environmental conditions in Australia north of latitude 19°S still favour the transmission of malaria were parasites to be reintroduced."
Of course, most of the US eastern seacoast south of New York would be considered malarial by the same definition.
It seems the WHO stated that parts of Australia with 19 degrees of the equator were vulnerable - no part of continental US, or for that matter Hawaii or Alaska, comes to within 19 degrees of the equator.
Having said that, the US (including parts North of NYC) did (and presumably could again) have endemic malaria (if parasites and the appropriate mosquitoes were reintroduced.) The was an outbreak as recently as the 1990's (on Long Island, NY).
