Paul Mulvihill left his wife for Rachelle Yeo after they both started work in 2007 at Sanofi Aventis Pharmaceutical company's Macquarie Park office in Sydney's north-west.

Ms Yeo was found with "critical injuries" at a first-floor unit in Pitt Road, Curl Curl, on Sydney's northern beaches before 9.30pm on Monday. Paramedics tried to save her but she died, police said.

Ms Yeo, 31, and Mr Mulvihill, 44, worked side-by-side at the company, he as the national pharmacy channel development manager and she as a hospital specialist representative.

An affair developed and became so strong that Mr Mulvihill eventually left his wife.

A friend of Ms Yeo told the Herald that when this relationship soured Mr Mulvihill was so distraught he became obsessive towards her.

"He would send emails, call," they said.

Ms Yeo met her new boyfriend Rob Hunt in Ireland when in 2005 they both worked for Dell computers. According to Mr Hunt's Facebook site they did not form a romantic relationship until recently.

"I'm very grateful for the short time I got to spend with her and I'm consoled in knowing that she was completely in love with me and in a very happy place in the end," he posted.

NSW Police confirmed they will allege a breakdown in Ms Yeo and Mr Mulvihill's relationship was a motive for her death.

Sanofi Aventis Director of Public Affairs & Communications, Alan Brindell, said in a statement the company was working with police.

"We are greatly saddened and shocked to find out one of our staff members from the Sydney office, Rachelle Yeo, has passed away in tragic circumstances.

"She was a wonderful person and will be greatly missed by all of us at Sanofi. We extend our deepest sympathy to Rachelle's family and are in contact with her family.

"We recognise the difficult nature of this news and are offering support and counselling services to all of our employees and their immediate families," he said.

Mr Mulvihill, who was arrested by Queensland police at Brisbane Airport yesterday after leaving on a flight from Newcastle, faced the Brisbane Magistrates Court this afternoon.

Magistrate Paul Kluck granted an extradition request by the NSW Police. Two NSW detectives were in court to escort Mr Mulvihill back to Sydney.

He was remanded in custody before his appearance at the Central Court tomorrow morning.

Mr Mulvihill will be formally charged with murder upon his arrival in NSW.

Outside court, defence solicitor Tim Meehan said his client would "vigorously defend himself" against any charges.

- with Amy Remeikis and Glenda Kwek