A LUCKY break for police in Victoria led to one of Australia's biggest drug seizures in Sydney, police said today.

Last night police seized half a billion dollars worth of liquid ecstasy from a warehouse at Castle Hill, in Sydney's northwest.

Their investigation began on December 18 when Victoria police officers stopped to help a broken-down hire van in Geelong and stumbled across $51 million worth of the liquid.

Two men, Patrick Pak Lamb Li, 50, of the Sydney suburb of Campsie, and Singaporean Keng Chuan Koh, 27, were arrested and charged with trafficking a commercial quantity of a drug of dependence.

They are in custody in Melbourne awaiting a court hearing on July 13.

Police said today the pair's arrest was critical to last night's drug bust in Sydney.

"Basically, it was the starting point for this investigation," Victorian drug detective Pat Boyle said.

Insp Boyle said the uniformed officers who stopped to help the driver of the van in Geelong were lucky.

"With any investigation there's always some luck," he said.

"Basically, the uniformed members were surprised at what they got.

"The major drug task force was called in ... and from there we started looking into it."

Methylenedioxyphenyl-2propanone, otherwise known as liquid ecstasy, is a final stage precursor for the production of ecstasy.

The Sydney find - 1900 litres of the chemical which could make $540 million worth of ecstasy - was welcomed by acting NSW Premier John Watkins today.

"This is believed to be one of the biggest drug seizures in this country's history," Mr Watkins said.

"I want to congratulate the NSW police who, with their Victorian counterparts, have dealt a major blow to drug dealers with this large drug haul."

No new arrests have followed last night's liquid ecstasy seizure, which police say could have produced 18 million ecstasy tablets within several days.

Police say the Geelong find - 340 litres of the chemical - could have made 1.3 million ecstasy tablets.

The combined busts prompted Australian Federal Police (AFP) Commissioner Mick Keelty to warn today that hype about the drug ice threatened to overshadow the larger problem of ecstasy and other drugs.

"Ice is not all of the amphetamine-type stimulants that are available and clearly ecstasy is one of the larger threats," he said.