Police began intercepting the telephone calls of Susan Dale Austen after discovering her relationship with a woman who committed suicide using a Class C drug.

The trial of Austen, 67, got under way in the High Court at Wellington on Monday.

The Lower Hutt woman has pleaded not guilty to three charges in relation to assisting a woman, Annemarie Treadwell, to commit suicide - two counts of importing Class C controlled drug pentobarbital on two occasions between 2012 and 2016, and one of aiding a person to commit suicide.

After jury selection, Crown lawyer Kate Feltham outlined her case, saying she would call 31 witnesses, including the police officers who attended the scene of Treadwell's death in June 2016, Treadwell's children, Customs officers who intercepted Austen's alleged importation of pentobarbital - a drug that taken in large quantities can suppress the central nervous system, causing a coma and death - and a vet who uses the drug to euthanise animals.

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Feltham says Austen's emails, police intercepts and Treadwell's hand- written diary would be key in making her case.

"Over a number of years, Ms Austen made contact with people in China and Mexico to arrange pentobarbital to be sent to New Zealand, and once picked up a package of the drug at a hotel in Hong Kong and brought it back to New Zealand," Feltham said.

She said a police investigation was begun after an autopsy showed Treadwell had pentobarbital in her system and a hand-written diary showing contact with Austen, who is the Wellington president of Exit International, an organisation that endorses euthanasia.

"We [the Crown] will show that Ms Austen gave Ms Treadwell an email address to contact a supplier to arrange pentobarbital to be delivered, which was intercepted by Customs.

"Police obtained access to Ms Austen's emails and saw conversations with Ms Treadwell and overseas [pentobarbital] suppliers."

Feltham said the day Austen was arrested, with a friend in a car park at Percy's Reserve, in Lower Hutt, she had in her possession two plastic bags containing quantities of pentobarbital.

The case is set down for three weeks.

- NZN