Hung Nguyen, of St Albans, was arrested getting off a bus at Southern Cross train station on Sunday morning with a suitcase containing zip lock plastic bags of a white coloured power.

Senior Constable Tim Norris told the Melbourne Magistrates Court Nguyen was among 30 passengers who got off the bus that had left Liverpool train station in Sydney at 8.05pm last night.

He said police were conducting an operation targeting the bus network with sniffer dogs.

One of the dogs indicated the presence of drugs in Nguyen's red suitcase, which had been stowed in the luggage area.

When questioned about the alleged drugs, Nguyen said she wasn't aware of their presence and thought her bags contained only clothing.

"This story appears totally untruthful," said Senior Constable Norris. "She was there for six hours, it's our belief that she's gone up there solely to transport the drugs back to Melbourne."

He said Nguyen's bus ticket had been purchased in a false name that matched a driver's licence the 26-year-old had in her handbag.

Her solicitor argued the evidence wasn't "clear-cut".

"She's very vulnerable, young, small and pregnant and we have no direct evidence that there was an illegal drug in her possession," he said.

Nguyen is three months pregnant and has lived in Australia for almost 11 years.

Two friends, one of 10 years and the other of three years, offered $10,000 sureties each if Nguyen was to be granted bail.

Magistrate Robert Kumar denied her bail application and remanded her to custody until a file hearing on December 14.

Nguyen cried as she was removed from the courtroom.