Justice Andrews found the 49-year-old could not prove how he paid for assets, such as a lifestyle block, 50ft launch and fleet of cars, while living off ACC payments for injuries he suffered in a motorcycle accident.
He claimed to have won large jackpots at SkyCity, but police analysis of the gambling records showed the cash spent - and cashed out - at the casino was more likely to be laundering of drug profits.
Duncan also said that the cars were owned by other people, or were sold shortly before seizure, while the boat was jointly owned in a syndicate of friends.
But Justice Andrews ruled in favour of awarding $2.4 million of assets to the Crown - the second largest forfeiture under the Criminal Proceeds (Recovery) Act which came into force in 2010.
The order included the forfeiture of a lifestyle block, 11 cars and cash totalling $514,000.
But $500,000 of that money was not found by police. Instead, detectives in Operation Safari learned the cash existed after the arrest of Duncan in September 2010.
A bugged conversation between Duncan and an associate after Duncan's arrest revealed a plastic bag of cash had been buried in the backyard. Police had dug holes in the lawn and had not discovered the stash. But one of Duncan's associates returned to the property one night after the police search to retrieve the money.
He had to "army crawl" and was "covered in mud head to toe" when he rang Duncan at 3am, according to the conversation transcript in Justice Andrews' decision.
"I think they've dug it up mate and put it back ... they have dug all around it, the whole property is ... dug up mate," the man told Duncan.
"They have dug right where that big lot was mate ... like a foot away ... but it's all still there."
Justice Andrews said the money was still missing.
Her ruling was suppressed until the end of the criminal trial to avoid prejudicing the jury against Duncan. He and another Tauranga man, Wallace Bramley, were acquitted of charges of manufacturing 2.3kg of methamphetamine and conspiring to supply the drug.
The jury was told that the pair had already pleaded guilty to other serious methamphetamine charges, including manufacturing $1 million worth of P. They are yet to be sentenced.
Estate agent Nigel David Walker, a former police officer, was found guilty of conspiring with Duncan to launder the profits of his drug enterprise.
Sworn affidavits from police in the High Court file also highlight Duncan's links to the Hells Angels and Filthy Few motorcycle gangs.
Previous methamphetamine and money laundering charges, laid against Duncan in 2007 when 200g of P and $60,000 cash were found in his car, were thrown out after the search warrant was successfully challenged.
Forfeiture 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". The onus is on defendants to prove how assets were paid for.
Seized from Duncan
*A 2.3ha lifestyle block worth $550,000
*11 cars, including a 1967 Chevrolet Impala and a Chrysler 300
*6 motorcycles, including a Harley Davidson
*2 boats, including a 50ft Spencer launch worth $140,000
*A 3.5 tonne digger
*Cash totalling $541,000
