The synthetic drug, carfentanyl, was intercepted by border force officers at a Brisbane mail centre and is about 10,000 times as potent as morphine.
The elephant tranquilliser is typically sourced from China, which announced a ban on it this week, and is so deadly to humans it can kill on contact.
Authorities won’t reveal how much of the drug was seized but are concerned about even the smallest amount making its way onto the streets given a dose as tiny as 0.002mg can kill.
“This is a dangerous drug. This drug will kill you,” Queensland Detective Superintendent Jon Wacker said on Friday.
“Our concern is also to the first responders who may turn up to assist.”
Carfentanyl, which is based on fentanyl, has been linked to thousands of deaths across the world in recent years, including in the US and Canada.
Queensland Clinical Forensic Medicine Unit director Dr Adam Griffin said suppliers were able to purchase the drug cheaply because they only needed small amounts and typically told buyers it was MDMA or ecstasy.
He said users typically died quickly.
“The actual substance itself may impair somebody’s breathing and stop it completely, (causing them) to lose consciousness, lose their blood pressure completely and die very shortly after ingestion or injection,” he said.
Even people who get the drug on their skin or in their eyes are at great risk of death.
A US department official last year told Associated Press the country was concerned carfentanyl could be used as a chemical weapon and was pressing China to blacklist it.
A former US defence department chemical weapons expert also told the news agency there were concerns about the drug being ordered by groups like ISIS.