Gib Bogle was a notorious womaniser, and he and Margaret Fowler had been having an affair since 1959. She was one of many women whom he'd slept with over the years. She became obsessed with him, and he tried to end the relationship several times, though still sleeping with her when he felt like it. Eventually Mrs Fowler left the CSIRO.
Later, Mrs Chandler caught Bogle's eye at the CSIRO Christmas barbeque, which would not have pleased Mrs Fowler. While the party was happening at the Nashs', Mrs Fowler was with her husband at another party several suburbs away. She had the motive for murder, and she knew that Bogle and Chandler were at the Nash party. She also bore a striking resemblance to Mrs Chandler, and may have been able to visit the Nash party undetected. However, it is generally agreed that she lacked the time to do this. She left the other party at 3:45am with her husband and went home. This gave her a little over 30 minutes to go home, then go to the Nashs', and without being detected, sneak in and poison Bogle and Chandler. Such a scenario is extremely doubtful.
Other speculation: