The group chose to open the package anyway and found a white powder inside wrapped in a piece of paper with the word "scoop" on it.
Nine backpackers were rushed to hospital in Australia after snorting a date rape drug they mistook for cocaine.
It is believed the drug, now identified as hyoscine, arrived in the mail posted from NY and was addressed to someone who had never lived at the residence.
The white powder that the nine snorted in Perth city on Tuesday has been identified as the common prescription medication hyoscine, which is used to treat various medical problems including travel sickness, Western Australia state health authorities said.
WA police said emergency services staff found "numerous people" in the house who were either unconscious or semi-unconscious.
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The other people affected - three French citizens, a German, Italian and Moroccan, all aged 21 to 25 - have been released from hospital.
Sydney woman Anna Catliffe has set up an online fundraiser to help the two Frenchmen still in hospital pay their medical fees.
The powder arrived in the mail, sent from NY and addressed to someone who had never lived at the house, in Victoria Park, WA.
A Royal Perth hospital emergency doctor, David McCutcheon, said the group might have died but for the fact that two flatmates who didn't take the drug were able to call 000, the paper reported.
Royal Perth Hospital clinical toxicologist Jessamine Soderstrom said overheating bodies were eventually fatal.
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One of the men returns to the property yesterday.
One of the backpackers, who returned to the house on Wednesday, told reporters he had felt like his face was "melting off" and that he had tried to scream but his mouth would not open.
Neighbors speaking to media described a frightening scene in which seven men and two women delirious and with "glassy eyes" were wheeled out on stretchers to nine ambulances.
The owners of the Colombo St house are being investigated by the Victoria Park council, due to it apparently being used illegally as a backpacker hostel.
Health Minister Roger Cook says he expects the hospitals will try to recover costs.
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