A banknote torn in half was the key item in a mid-ocean rendezvous that led to Australia's largest-ever methamphetamine seizure — more than a tonne of the drug worth an estimated $1 billion, the WA Supreme Court has been told.

The detail emerged at a sentencing hearing for Joshua Joseph Smith, who has pleaded guilty to involvement in the importation of the drug into the port city of Geraldton in December last year.

He was one of nine men charged after authorities seized the drug as it was unloaded from a boat called the Valkoista and loaded into a van.

Officers have removed a number of packages from the boat which was brought into the Geraldton marina. December 21, 2017 ( ABC News: Jessica Hayes )

In a statement read by Commonwealth prosecutor Patricia Aloi, the court was told the charges were the culmination of an multi-agency investigation that had started five months earlier and involved the covert surveillance of some of those alleged to be involved, including Smith.

Ms Aloi said Smith, who is from New South Wales, and others involved were detected flying and driving across the country, as well as to and from Geraldton at various times, while intercepts revealed conversations which included talk about "some bags [that] need hiding".

Map drawn on hotel slip

Another piece of evidence was a map of Australia and South East Asia drawn on the back of a check-out slip from Perth's Crown Towers hotel, where some of the alleged participants in the importation were seen in September 2017, meeting and dining at an upmarket restaurant.

The court heard the map had arrows on it pointing to the east and west coast of Australia.

A hand-drawn map of Australia and South-East Asia was drawn on the back of a check out slip from Perth's Crown Towers. ( Supplied )

Intercepts also recorded the mid-ocean rendezvous about 300 nautical miles west of WA, when the drugs were handed over from a "mothership" to those on board the Valkoista on about December 19, 2017.

Ms Aloi said an Asian voice was heard saying "money, money" before a "token" — a half-torn banknote — was shown by each party to confirm their identities.

As the Valkoista made its way back to shore, further intercepts revealed messages between those on board and others back on land, such as "ETA please", "everything OK bros" and "please respond because I need to jump on a flight to meet you".

In the early hours of December 21, the boat was tied up at a jetty in Geraldton before a rented van drove up and 59 bags, containing almost 1.2 tonnes of methamphetamine, were loaded into it.

A total of almost 1.2 tonnes of methamphetamine was seized. ( ABC News: Nicolas Perpitch )

The court heard the van and the boat were then intercepted by authorities as the drug smugglers tried to leave the scene.

An analysis of the drug revealed that the methamphetamine had a purity of between 79.8 and 80 per cent.

Court closed for submissions

Smith is facing a maximum of life in jail.

However, the court was closed to the public and the media for submissions by his lawyer.

Justice Joe McGrath said the decision was made "for reasons of public policy".

Smith will be sentenced later this week, but it is likely the court will again be closed to the public and media.

Smith has been in custody since his arrest in December 2017.

One of the eight other men has also pleaded guilty and is due to be sentenced later this year, while the remaining seven are yet to enter pleas to the charges against them.

A van intercepted by Australian Federal Police officers in Geraldton contained a record 1.2 tonnes of methamphetamine. ( Supplied: AFP )

Editor's note June 27, 2018: An earlier version of this story incorrectly referred to this operation as Australia's biggest drug bust. It was the country's biggest methamphetamine bust, but not the biggest overall drug bust.