The pair drove in a Nissan ute to the counselling session together and left at 12.15pm.
Their ute was spotted parked outside a farm in Te Kuiti for four hours and at 5.30pm Wallace was seen hopping back in the vehicle before they both drove back to his Otorohanga home.
Once back at the property, Wallace was sitting in the driver's seat when he fired a rifle at Leigh Wallace, who was sitting beside him, hitting her in the head.
Wallace then called a family member and told him he "had done something stupid".
Family members arrived within 15 minutes to find blood coming from the grandmother's mouth. They grabbed the gun off Wallace.
When police arrived and asked Wallace if he had shot his partner, he nodded. He was unable to speak to police because of his injury.
Emergency services arrived and confirmed Leigh Wallace was dead.
Wallace is due to be sentenced on December 13.