Troy Dean Waite, 29, waited outside the Stratford shop for the two to exit, calling abuse to them as he lingered.
When the victim walked out, Waite punched him in the head, causing the man to drop his two drinks, the New Plymouth District Court heard on Friday.
But Waite hadn’t finished assaulting him. He grabbed the victim’s shirt, pulled him downward and punched the man’s face a further four times.
The victim tried to fend off his attacker with two punches and then fled the scene.
When police caught up with Waite, he told officers he believed the victim was seeing his ex-partner and that the June 4 “fight” was consensual.
In court, Judge Chris Sygrove said he got the impression Waite and the woman had since “reconciled to a certain extent”.
The pair were expecting a baby together, it was heard.
While Waite had a “chequered history”, Judge Sygrove said it appeared he was now ready to take stock of his life, and to be a father and “not some sort of criminal loser”.
His “head exploded” when he thought he had witnessed an “illicit relationship”, but Waite was remorseful for his actions and receptive to rehabilitation, the judge said.
The mother of the child provided the court with a letter saying she was not afraid of Waite but would like him to seek help.
As a registered social worker, she was aware of the signs of domestic violence, she wrote.
Waite was now involved with two men’s counselling services, it was heard.
On an admitted charge of common assault, Judge Sygrove sentenced him to 18 months’ intensive supervision and ordered he pay the victim a $400 emotional harm payment.