(Reuters) -One of Australia's top IVF providers mistakenly implanted a patient with her own embryo instead of her partner's in a second fertility clinic mix-up, heightening concerns about an industry that did not have much active government oversight until recently.

Monash IVF ( MNIVF ) said the error took place on June 5 at a clinic in Melbourne but did not provide further details such as how it learned of the bungle or what the couple planned to do next. The company said it was supporting the couple, who it did not identify.

It said the patient's embryo was mistakenly implanted under a treatment plan which called for an embryo from the patient's partner to be transferred.

The incident builds on a reputational maelstrom for Monash IVF ( MNIVF ) which was already reeling from an April disclosure that an Australian woman had given birth to a stranger's baby after a fertility doctor accidentally implanted the wrong embryo to a patient in Brisbane in 2023.

That mix-up sparked concerns about security protocols at IVF clinics and an industry which is only now in the process of being more regulated.

Monash claimed the world's first IVF pregnancy five decades ago and is Australia's second-largest IVF provider, carrying out nearly a quarter of the country's 100,000 assisted reproductive cycles a year, according to industry data.

"This mix-up, the second reported incident at Monash IVF ( MNIVF ), risks shaking confidence not just in one provider but across the entire fertility sector," Hilary Bowman-Smart, a researcher and bioethicist at the University of South Australia.

Shares of Monash IVF ( MNIVF ) were down 24% by midsession on Tuesday, against a rising broader market. The stock is just over half its value before the April announcement.

"We had thought the Brisbane clinic embryo transfer error was an isolated incident," Craig Wong-Pan, an analyst at RBC Capital Markets, said in a client note.

"We believe there is now risk of a greater impact of reputational damage and market share losses to MVF's operations."

Monash IVF ( MNIVF ) had already hired a lawyer to run an independent investigation after the Brisbane incident, and said on Tuesday it has extended the scope of that investigation. It added that it was installing interim extra verification safeguards to ensure patient confidence.

"Whilst industry-leading electronic witness systems ... are being rolled out across Monash IVF ( MNIVF ), there remains instances and circumstances whereby manual witnessing is required," the company said.

It said it had reported the Melbourne incident to state regulator the Victorian Department of Health and industry licencing body the Reproductive Technology Accreditation Committee (RTAC), part of industry group the Fertility Society of Australia.

The health department and RTAC were not immediately available for comment.

Reports of transferring a wrong embryo are rare, according to fertility experts, and Monash's Brisbane mix-up was widely reported as the first known case of its kind.

($1 = 1.5330 Australian dollars)