The then 45-year-old was arrested at his Sydney home in February 2021 a month after crystal methylamphetamine was found concealed in eight candles.
With each candle containing 500 grams of the drug, police estimated the total haul could have sold on the streets for more than $2 million.
Police raids on a number of premises led to other arrests and the discovery of one kilogram of heroin, worth $250,000, and $75,000 cash.
Miller pleaded guilty in Sydney's Central Local Court on Tuesday to two counts of supplying a commercial quantity of a prohibited drug.
A charge of knowingly directing a criminal group was withdrawn, while he has admitted participating in a criminal group.
A charge of dealing with the proceeds of crime, less than $100,000, is to be transferred to the sentencing court.
He was remanded in custody and the case adjourned for first mention of a sentence hearing on April 8 in the Downing Centre District Court.
Police previously said the drugs, from Sydney, were bound for regional NSW.
Miller won silver and bronze at the 1996 Atlanta Olympics and was briefly married to Sydney model Charlotte Dawson.
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