Police seized 766kg of MDMA powder in the northern Australian state of Queensland on Tuesday.
The powder, the highest purity recorded in Queensland, could have made up to 12 million pills with an estimated value of £50.7m, police said.
Detective Superintendent Jon Wacker said the drugs represented around 7% of Australia’s yearly total – the biggest MDMA seizure in Queensland and the third-biggest in Australia.
He added: “Organised crime syndicates are making significant profit at the expense of our Australian community but more importantly they are placing our young people’s health at risk.
“This operation has been focused on targeting and dismantling this organised syndicate.”
Two Britons, a 51-year-old man and a 40-year-old man, were arrested on suspicion of drug supply and possession. They are being held in custody.
An Australian man, 26, was also arrested on suspicion of possession and supply of dangerous drugs while being a participant in a criminal organisation.
The drugs were found during raids on properties in Loganlea, south of Brisbane.
Police also searched properties at Lennox Heads and Ballina in northern New South Wales.
Vehicles and around AUS$108,000 (£61,000) in cash were also seized by police.
It comes just days after police in the New Zealand city of Auckland seized 200kg of methamphetamine during a search of an apartment.
Two Britons have been charged in relation to this – a 60-year-old man arrested at the apartment and a 49-year-old man at the city’s airport.
The bust was one of the largest of its type in New Zealand and the drugs would have been worth tens of millions of dollars on the street, police said.
An Australian woman, 51, was charged in connection with the money seized.
There is also an arrest warrant for a Australian man, 42.
In New Zealand, Detective Inspector Paul Newman said: “We’re making inquiries with police liaison officers overseas and hopefully tapping into our law enforcement partners overseas to try and work out the threat that this syndicate poses to New Zealand.”
In Australia, Det Sup Wacker said police are working closely with crime agencies in the UK and believed they had “disrupted this crime syndicate’s Pacific region operation”.