Story highlights The drugs are blocks of methamphetamine and cocaine, police say

A U.S. citizen and a Canadian have been charged with drug-importing offenses

The steamroller was tracked down to a storage unit in northern Sydney

Australian police have seized as much as $246 million worth of drugs stashed inside a steamroller sent from China after busting an international smuggling ring in Sydney.

A U.S. citizen, 33, and a Canadian, 34, have been charged with drug-importing offenses in relation to the confiscated methamphetamine and cocaine, Australian Federal Police said Wednesday. The charges carry a maximum penalty of life imprisonment.

The operation to seize the drugs resulted from a "complex and protracted" investigation over the past two and half years, police said.

The steamroller was shipped in from China, police said, and was tracked down at a storage unit in northern Sydney amid a series of searches. When officers searched the unit, they found the vehicle, dismantling equipment and 13 sports bags containing "blocks of compressed powder and a crystalline substance."

The blocks consisted of 235 kilograms (520 pounds) of methamphetamine, with an estimated street value of up to 174 million Australian dollars ($180.2 million), and 115 kilograms of cocaine, with an estimated street value of up to 63 million Australian dollars ($65.3 million), police said.

The Australian Federal Police cited experts as saying that data shows rising use of both drugs in the country.

The seizure comes less than a week after authorities in the Pacific island nation of Tonga found cocaine with a potential street value of as much as 116 Australian dollars ($120 million) in a yacht that had run aground off one of the country's northern islands.

That discovery was the result of cooperation between Australian, U.S. and Tongan authorities, Australian police said.