Thousands of Australians have received a "death threat" text, demanding they pay 5,000 Australian dollars ($5,140, £3,311) or face being murdered.
The scale of the scam has surprised the police authorities.
At a press conference in Queensland, Det Supt Brian Hay said: "Do not respond. Delete it immediately and don't panic... because that's what they prey upon."
The fraud is believed to be the work of an organised crime gang.
The message, which began to hit people's phones on Monday, reads:
"Sum1 paid me to kill you. Get spared, 48hrs to pay $5000. If you inform the police or anybody, death is promised."
It directs people to a Yahoo email account which police have now disabled.
Mr Hay told reporters that enquiries were ongoing as to whether the criminals were based in Australia.
Some people had already fallen for the scam, mainly those with little experience of text messaging, he revealed.
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