The plumber, 46, couldn't stop laughing as he said how 'hideous' Ronnie and Georgia's outdoor mural was.
'Oh my God, they've just lost half a million dollars,' Jason said to the camera, as the team eagerly tried to finish their backyards before the reveal.
'That's for train stations,' he said, as a painter drew white birds on a brick wall.
'I actually find it cheapens it to me...I wouldn't want that in my backyard,' he said.
Georgia said how excited she was for the mural, as she and Ronnie had been planning it since the start of The Block.
'It's not street art,' she said to the camera.
On Sunday's show the teams scrambled to finish their backyard before reveal. The pools and spa were installed, while they were rushed to get pool certification so that they could fill their spaces with water.
The judges were impressed with the backyards this week, especially Josh and Elyse's space, with the pair winning that week's $10,000 prize.
Jason and Sarah finished on 27 points, while Ronnie and Georgia finished on 25.5 points and were told to make their backyard more family-friendly.