Peter Nash helped run the chat forums on the site that let people buy and sell illegal drugs and other illicit goods and services.
The Silk Road was shut down in October 2013, following raids by the FBI and other law enforcement agencies.
Mr Nash could face life in prison when he is sentenced in May.
Arrested in Brisbane in December 2013, Mr Nash was extradited to the US in November 2014 to face trial for his alleged role in running the Silk Road site.
The Silk Road was hosted on the Tor network to help hide it from the authorities and to make it easier for people to use anonymously.
The US Department of Justice alleges that the founder of the Silk Road, Ross Ulbricht, paid Mr Nash up to $75,000 (Â£50,000) a year for administering the discussion forums on the site.
The forums on the Silk Road, said the DoJ, had helped visitors to the site find and buy drugs and other illegal services.
Over the two and a half years that the Silk Road was running, it had helped drug dealers sell hundreds of kilograms of their wares to more than 100,000 buyers, said the DoJ.
In addition, said the agency, millions of dollars had been laundered via the site, which only accepted payment in bitcoins.
Mr Nash, known online as samesamebutdifferent, said he had become involved with the site when buying drugs for friends.
"I deeply regret my conduct and any consequent harm I caused," said Mr Nash at his hearing.
He added that he had never found out the real identity of Ross Ulbricht, who founded the site.
In February this year, Ulbricht was found guilty of conspiracy to traffic drugs and launder money as well as hack computers.
He is planning to appeal against the verdict.
Two other alleged former Silk Road staff members are also facing trial. Andrew Jones pleaded guilty on 2 October 2014 and is due to be sentenced later this year.
Gary Davis is currently on bail in Ireland awaiting the conclusion of extradition proceedings.
An Australian man who helped run the underground Silk Road website has pleaded guilty to money laundering and drugs trafficking charges in the US.