More than 11 kilograms of pure powdered fentanyl and 30 kilograms of methamphetamine were found inside an industrial wooden lathe at the Port of Melbourne in December last year.
It took authorities two weeks in February to safely remove and analyse the drugs, which were sent from Canada.
Fentanyl is a fast-acting opioid that is highly addictive and acts similarly to heroin.
A single dose of 28 grams can kill a person.
"We were absolutely shocked by the size of the detection and the audacity of this attempted importation," Australian Border Force Commander James Watson told reporters on Monday
"I describe it as a total act of bastardry, quite frankly."
Fentanyl is primarily used for medical purposes in Australia but it's becoming used more prominently overseas for illicit reasons.
Australian authorities have previously seized only minor amounts of fentanyl, totalling less than 30 grams.
The Australian Federal Police are yet to charge anyone in relation to the December intercept.
"There is someone out there in the community who does know more," AFP acting Commander Anthony Hall told reporters.
"We're appealing to that person or persons to speak up and call Crime Stoppers."
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