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After spending nearly $11 million at SkyCity in six months, it's little wonder Tac Kin Voong was nicknamed "Mr Casino".

An unemployed painter, the Vietnamese national was under covert surveillance by police between July and December 2006, when he spent most of his time in the VIP gaming lounges.

He was convicted last week on drugs charges after a trial in which Crown prosecutor Mina Wharepouri told the court that Voong used the casino as an office, making phone calls to suppliers and arranging for cash and P to be exchanged.

The amount of money is staggering. Voong alone had a turnover of nearly $11 million at SkyCity in just six months. After bugging the phone of Voong's lieutenant Yihua Luo, Operation Ice Age detectives found another $2.5 million in bundles at a foreign exchange shop across the road from the casino.

That money has been seized under the Proceeds of Crime legislation.

Rival syndicate leader Ri Tong Zhou was also running a multi-million P ring from SkyCity and was also gambling the profits to launder the money.

He spent $8 million in the same six-month period and was later jailed for 15 years as the ringleader in Operation Manu after admitting more than 30 methamphetamine-related charges.

By spending so much money, the Asian underworld figures were able to "wash" the profits of drug dealing by laundering the money through the casino.

This can be done several ways. One is simply exchanging cash for casino chips, then cashing them in later in the day for a winner's cheque, although doing this too often can raise suspicions among casino staff.

Alternatively, money can be pumped straight into poker machines, which work on a mathematical calculation that the casino will eventually keep 12 per cent of the takings.

That means that by spending enough time - and money - someone can recoup up to 88 per cent of the money gambled.

Voong hired people to play poker machines all day to do this.

Voong and Zhou were the central targets in two separate police investigations in 2006 that were unaware of the other inquiry until the pair began selling methamphetamine to each other.

The pair were seen rubbing shoulders in the VIP room and it can be now be revealed that the drug lords had been dealing with each other when drug supplies were low.

While under surveillance by police for months, Voong and Zhou spent most of their time in the casino where they would speak on the phone to suppliers and dealers and arrange meetings for cash and drugs to be exchanged.

Sending Zhou to 15 years in prison, Justice Rhys Harrison said that Zhou had used the casino as "offices" and noted that anti-gambling lobbyists had warned that the casino could become a "scene for large-scale criminal activity or a meeting place for people who commit serious crimes".

Zhou supplied millions of dollars of P to Tauranga man Alan McQuade, who then distributed it to his criminal contacts in motorcycle gangs in the Bay of Plenty.

Lyrice Peri did the same with her network in the Waikato.

Likewise, bugged conversations between Voong and fellow Vietnamese Hung Trung Kha discussed money being taken to Voong's girlfriend's place.

Detective Sergeant Mike Beal told the court how another detective squad searched Kha's car on a Remuera street in October 2006. They found a pipe, 80g of P, $95,000 cash - and gangster Daniel Crichton.

A former Black Power member with links to the Headhunters, Crichton helped with the safe return of the stolen Waiouru war medals.

For his part, the 40-year-old received bail while on the drugs charges with Kha and got a nine-month discount on his eventual jail term. Before his imprisonment, Crichton and underworld kingpin Waha Saifiti denied rumours that Crichton was a "nark" who told police who the medal thieves were.

The pair also distrusted Kha who pleaded guilty to more serious methamphetamine charges.

In July 2007, Internal Affairs admitted casinos had a problem with organised crime.

A report made public by then-Minister of Internal Affairs Rick Barker revealed the agency and police had "a significant amount of evidence" that predominantly Asian groups "were frequenting NZ casinos for the purposes of criminal networking, money laundering and social gambling using illegitimate funds".

SkyCity's general manager for Government and industry affairs, Peter Treacy, said the casino did not tolerate criminal or "undesirable" behaviour SkyCity had a strong relationship with the police and the Department of Internal Affairs, particularly in sharing surveillance film and information to identity criminal activity.

"If the public are aware of illegal or undesirable behaviour in our casinos, we would strongly encourage them to contact us or the police."