Rita Cappelleri, who turned to cannabis oil in recent months after a battle with breast cancer, was boarding the ship at Circular Quay when her oil was detected by security and handed over to customs.

Despite desperate pleas from Mrs Cappelleri and her supporters, she was denied the medicine for the nine-day trip.

It comes just weeks after Premier Mike Baird promised legitimate medical marijuana users would be exempted from prosecution while an expert working party investigates legalising the drug as medicine in NSW.

Tamworth medical marijuana champion Lucy Haslam said Mrs Cappelleri's experience made a mockery of that promise.

"There needs to be communication from the government, and while police might be aware (of the exemption), clearly Customs aren't," Mrs Haslam said.

"This is a great example of why these laws need to be sorted out quickly."

Mrs Cappelleri, who could not be contacted for comment, spoke candidly about her experience with cannabis oil to ABC's Lateline program last month.

She described the chemotherapy as "un- bearable" before she discovered the benefits of cannabis oil.

"I started it about three days before I had my first chemo and I just breezed through it. I had my first chemo, had my second, third I felt absolutely nothing," she said.

"I'm a great believer in it now ... and I have put other people onto it."

NSW Hemp Party president BJ Futter, who has been advising Mrs Cappelleri, said the cannabis oil had helped control the size of her tumours and greatly enhanced her quality of life.

"I'm outraged that she could be treated this way and that her medicine could be confiscated," he said.

"Everyone has a right to quality of life and everyone has a right to choose their own medicine.

"There should be an immediate amnesty put on for people that are using cannabis oil as medicine because they have no other option."