He said he had been drinking alone in an area near the family’s Wangaratta property in northern Victoria when he came home to find his mother dead in the shed.
Thomas said he armed himself with a pickaxe handle and went inside where he was confronted by his father, who had a single-barrel shotgun.
He said he managed to wrestle it from his father, who he then shot to save his own life.
The jury rejected his story.
They had heard no evidence Mr Thomas had ever been violent towards his wife.
On Thursday, they found Thomas, 38, guilty of two counts of murder over the deaths of Pauline, 63, and William Thomas, 65, on April 21, 2013.
By their verdict they were accepting the Crown’s case that Thomas strangled his mother then lay in wait for his father.
Experts said the gun was fired at Mr Thomas from four to four-and-a-half metres away.
He then caved in his father’s skull with the handle of a pick-axe.
Thomas put the cable ties he had used to strangle his mother into a bag along with the pick axe handle, the gun and his own bloodied motorbike gloves.
He drove to a Geelong pub where he met his married lover Jacinta Emselle, and confessed.
He was drinking a beer and reading a paper, Ms Emselle said during the trial, and had a ‘smirky’ look on his face.
When she enquired he replied: ‘Let’s just say that my parents are no longer with us’.
Thomas said he had argued with his mother over a hair dryer and strangled her, she said.
They then disposed of the murder weapons.
Thomas didn’t call the police because, according to his evidence: ‘I thought they would blame me’.
During the trial he painted his father as a violent and abusive man.
Neighbours and friends described William and Pauline Thomas as a harmonious couple.
Thomas did not react after the jury delivered the verdict of guilty on two counts of murder.
He was remanded into custody and will return to court for a plea hearing on a date to be set.