Two others, William Wolfgramm and Angus Naupoto, had pleaded guilty earlier.
The group thought they were dealing with a Tongan supplier who had stolen the huge stockpile.
At the time of the arrests, Detective Inspector Bruce Good said the Tongan "drug syndicate" had gone to great lengths to convince the potential buyers in New Zealand that the product was genuine.
"They made blocks to look like a [methamphetamine] stockpile then put rock salt on it," said Mr Good. "On top of the rock salt, they put a small amount of locally purchased methamphetamine.
"So when [the buyers] asked for a sample, they gave them a shard of genuine ice, methamphetamine."
By this time, police in Tonga and New Zealand were investigating the conspiracy, and were watching as Naupoto waited for a fishing vessel carrying what was believed to be 20kg of methamphetamine to arrive.
But police thought a woman was being held hostage to ensure the safe delivery of the "drugs" and decided to move in when threats were made against her.
Detectives discovered that no methamphetamine existed and the hoax was an attempt to scam hundreds of thousands of dollars from the New Zealand group.
Wolfgramm is a former Tongan league international who played at two World Cups.
Naupoto is the former manager of the Tongan rugby union team and secretary for the Tonga Rugby Union, and Tanginoa is a former Tongan union international.
Mafi is a King Cobras gang member.
The players:
William Wolfgramm and Angus Naupoto pleaded guilty to conspiracy to import methamphetamine
Peter Lawrence Tanginoa found guilty of conspiracy to import methamphetamine
Douglas David Afeaki and David Charles Mafifound guilty of conspiracy to supply methamphetamine
Read more: Crime syndicates target islands