A 32-year-old man and a 31-year-old woman were arrested on Monday by Australian Federal Police (AFP) and Australian Border Force (ABF) officers.
The arrests are part of an AFP investigation launched on October 6 after ABF officers in Perth intercepted the package, which was sent from the US.
Authorities cut open the blue plastic tub to access the illicit drugs.
They said an x-ray showed inconsistencies and two drug sniffer dogs also flagged the package.
Testing on the thick black paste returned a positive result for methamphetamine.
AFP officers changed out the intercepted container for one without illicit drugs.
Police said the woman allegedly collected the substituted package from a short-stay apartment in Scarborough where it had been delivered.
She then met with the man before they took the package to a home in Willetton.
The pair were arrested as they tried to drive away from the house.
Police searched the home and allegedly found black paste from the container in a pile on the bedroom floor.
AFP Acting Sergeant Mat Glare said investigations continue into the drug importation, with hopes to find others involved.
“Methamphetamine is a highly addictive drug, which can directly lead to violence and mental health issues,” Acting Sergeant Glare said.
“The demand for it results in property crime and other offences by addicts trying to fund their next hit.”
If sold, the 18 kilograms of methamphetamine could have been sold at street level for approximately $18 million.