At least 30 people were arrested in police raids in December 2013 but Yang was overseas at the time. The 53-year-old has not returned to face the criminal charges, but the police obtained a court order for his property to be forfeited in a separate civil case.
This includes a mortgage-free home bought for $1 million and several bank accounts, including one with $504,000.
"The inference available is that his assets would have been funded, at least in part, through criminal activity," according to the police application on the court file.
In his judgment released this week, Justice Edwin Wylie agreed.
"The evidence gathered by the police indicated that during much of 2013, Mr Yang was a very substantial wholesaler of pseudoephedrine in the form of ContacNT granules," the High Court judge wrote.
"It appeared that he may also have been involved in pseudoephedrine dealing in New Zealand from about 2009."
Justice Wylie said Yang had a "modest income, but he spent substantial sums of money at SkyCity casino and he was living well beyond his means".
In the 19-month period leading up to the Operation Ghost raids, Yang had a "buy-in" figure of more than $2 million and a gambling turnover - the total of money spent and winnings - of more than $14 million at SkyCity. He earned just $4217 over the same period, or $222 a month, according to Inland Revenue documents on the High Court file.
Intercepted phone communications indicated his partner, Chao Ying "Anna" Chen, took control of the drug dealing after Yang left the country.
"Again, she had a modest income. Indeed she was in receipt of state funded benefits," said Justice Wylie, which totalled $34,000 over three years. "However, there were significant assets under her control, including thousands of dollars in bank accounts." Chen owned two mortgage-free homes which have now been forfeited.
Justice Wylie believed the assets were the profits of criminal activity.
"I am also satisfied that the proposed settlement, involving forfeiture of almost $4.5 million in assets, appropriately recognises the strength of the [Police] Commissioner's case for forfeiture."
The Weekend Herald has previously revealed at least six other Operation Ghost targets spent millions of dollars at the casino.
A spokesman for SkyCity declined to comment on the latest ruling. But in a previous statement about Operation Ghost, the spokesman said the casino had co-operated fully with the police and the Department of Internal Affairs and "fully discharged" its legal responsibilities, which he could not elaborate on because of legal restrictions.
"We take our responsibilities in this area very seriously. We place a very strong emphasis on surveillance and security, spending millions of dollars on this each year. We work very closely with the police and other enforcement agencies."
The assets were restrained under the Criminal Proceeds (Recovery) Act.
The law essentially forces someone to prove how an asset was paid for. These cases are determined by the civil level of proof, the "balance of probabilities", rather than the much higher criminal evidential threshold of "beyond reasonable doubt".
Forfeited property
Zhi Wei Yang
Alleged to have supplied pseudoephedrine, but avoided arrest as he was overseas. He has forfeited $4.5 million of "tainted" assets to police including:
• A mortgage-free home purchased for $1 million and three other houses.
• Bank accounts holding $79,000, $36,000, $72,000 and $504,000.
• More than $25,000 in cash.
• SkyCity Casino records show Yang had a gambling turnover of more than $14 million in 19 months. Over the same period, he earned $4217, according to Inland Revenue records.
• His partner at the time, Chao Ying "Anna" Chen, received social welfare payments of $34,000 between 2009 and 2012.
