Two men have been arrested in Belgium in connection with Australia's biggest seizure of the drug ecstasy.
The ecstasy tablets were found by X-ray machines
Australia's police commissioner said a major European syndicate was behind the shipment, and more arrests were likely.
Two other men were arrested in Sydney on Sunday after police found 3m ecstasy pills in a bakery oven imported from Poland, via Germany.
The two appeared in court on Monday charged with conspiring to import and attempting to possess banned goods.
Australians Zachariah O'Brien and Robert Drury were arrested after taking delivery of the oven and allegedly removing its contents in a factory in Sydney.
They face lengthy prison sentences if convicted.
A police spokeswoman said on condition of anonymity that the two men arrested in Belgium were European, but did not specify their nationalities.
Australian Federal Police Commissioner Mick Keelty said both men were "higher up the food chain".
"We anticipate that there will be further arrests in Europe, but of course we are working as best we can with the overseas authorities to maximise their opportunities to make those arrests," he said.
In all, 62 bags were found containing about three million pills and 115kg (254 pounds) of MDMA powder, the active ingredient in ecstasy.
The haul was said to be worth about $200m (US$154m).
"Most of the MDMA we get in Australia is in fact sourced from Europe," said Australian Federal Police Commissioner Mick Keelty.
"In this particular case the air freight came directly from Germany to Australia but we believe the origins are in Poland."
Mr Keelty said there might be a link between the Sydney haul and a thwarted attempt to smuggle ecstasy to Adelaide in a pizza oven earlier this year.
Australian Federal Police agent Mike Phelan said the latest find would substantially dent ecstasy supply in Australia.
"There is no doubt we have disrupted a very sophisticated and significant syndicate of international drug traffickers," he told Australian Broadcasting Corporation radio.
"Overseas, we have got some further inquiries being carried out on our behalf by the Belgian and Dutch authorities."