Judge Peter Hobbs remanded McMahon in custody until sentencing on November 16.
McMahon is alleged to have sold cannabis and laundered money from its proceeds over a seven year period between January 2005 and her arrest in June this year.
Police seized more than 13kg of cannabis and about $40,000 in cash at the end of a five month investigation dubbed Operation Foxy.
Four others were arrested as part of the operation, including McMahon's 48-year-old-son Craig McMahon.
Wellington organised crime manager Detective Inspector Darrin Thomson said at the time that Operation Foxy had severely impacted the cannabis supply chain in the Wellington region.
"This syndicate was responsible for millions of dollars worth of drugs flowing into our community and causing damage to many," he said.
Mr Thomson said the group had been operating for a number of years and built up millions of dollars in assets from the proceeds.
"We have been gathering evidence of the assets accumulated by this syndicate over a period of time and our asset recovery unit is in the process of restraining several million dollars worth of residential property and cash," he said in June.