He will be sentenced next month and was remanded in custody by Justice Timothy Brewer.
Two detectives flew to London to get McGannan, and he arrived in their custody at Auckland International Airport in October.
Five people were arrested in Auckland, Wellington and Christchurch during Operation Gringo last December 2011 and four entered early guilty pleas.
Detectives from the Organised and Financial Crime Agency allege McGannan was connected to a Mexican drug runner who smuggled cocaine to contacts in New Zealand.
The 3kg seized was hidden in a suitcase belonging to David Negrete Nevarez, who landed at Auckland Airport on a Lan Chile flight from Santiago in December, 2011.
Nevarez, 43, pleaded guilty to importing and possession for supply of the Class A drug, and was sentenced to seven years in prison.
Covert surveillance led to four other arrests around the country, two of them of contractors helping to rebuild Christchurch.
A "controlled delivery" of the cocaine was followed to Auckland woman Samantha Margaret Gemmell, 27, who then passed the parcel to Adrian Marquiss Kemp, 31, in a Mission Bay carpark.
Gemmell and Kemp each admitted a charge of possession of cocaine for supply and were jailed for two years and six months, and two years 10 months respectively.
Under police surveillance, Kemp transferred the drug package to Brendan John Clarke in Wellington.
Clarke agreed to find buyers for the 80 per cent pure cocaine after the original purchaser pulled out.
Police swooped when the Wellington man opened the suitcase to inspect the package.
The 37-year-old pleaded guilty to possession for supply and will spend four years and eight months in prison.
Clarke was a self-employed builder with 16 staff, and worked as an Earthquake Commission contractor inspecting damaged Christchurch homes.