Pharmacist Yan Chi "Anthony" Cheung started making sexual advances towards Pamela Leung when she began working with him.

An Australian pharmacist has admitted to drugging his colleague 23 times with prescription drugs because she rebuffed his sexual advances.

On Tuesday, Yan Chi "Anthony" Cheung pleaded guilty in Waverley Local Court in Sydney to one count of poisoning to injure or cause distress or pain.

Cheung, 35, and his victim, Pamela Leung, 26, have been friends since they met at the Surry Hills Chinese Presbyterian Church in 2010.

Pharmacist Yan Chi Cheung outside Waverley Local Court in Sydney on Tuesday.

In 2015, Cheung encouraged her to take a position at Pharmacy@UNSW, based on the University of New South Wales campus, and started making sexual advances towards her when she began working with him.

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Cheung, who is married, brushed past her breasts, buttocks and hands, according to a police statement of facts tendered in court.

"The victim felt as though [Cheung] became obsessed with her," the police statement said.

When Leung confronted him about his behaviour, Cheung stopped.

However, he then started putting prescription drugs in her water and her coffee to make her "suffer" for refuting his advances, he told police when he was arrested.

Over the course of a year, he drugged her 23 times using phenergan, doxylamine, endep, seroquel and deptran.

At first, he crushed two 25 milligram tablets of phenergan into her water three times in three weeks.

At one stage, around March 2016, he put 150 milligrams of deptran into Leung's tin of instant coffee meaning she poisoned herself each time she made a coffee.

The drugs made Leung extremely drowsy to the point where she had to take naps in the pharmacy storeroom or return home where her husband would find her unresponsively asleep.

Some of the drugs can also cause infertility over prolonged use.

"As a pharmacist, [Cheung] had detailed knowledge of these drugs and the effects through his studies and employment," police said.

Leung eventually looked at CCTV footage from the pharmacy when she noticed her water tasting odd. She had also noticed Cheung's behaviour change.

Cheung was initially accused of trying to tamper with the CCTV footage to erase it or change the time codes however this charge was dropped when he agreed to plead guilty.

Cheung, who required a Cantonese interpreter in court, will be sentenced on June 5 after a pre-sentence report is completed, including a psychological assessment.

A spokesman for UNSW said the pharmacy is an external retailer on campus and workers are not employed by the university.

"We are advised that the pharmacist involved has had their employment terminated by the retailer and reported to the Pharmacy Guild," the spokesman said.