Groves and his partner had intended to sleep in their car at the stadium, but Groves had been drinking and an argument erupted.
Police prosecutor Graham Hoskin told the court Groves punched his partner "a number of times to the head and face, causing her to put her hands up to block his blows."
The pair had been together for two years and in that time there had been 10 family harm incidents between them.
"The vehicle's door was open on her side, she called for help and young children witnessed the assault," Hoskin said.
"As the victim faced out towards the children, the defendant punched her in the back, hitting her repeatedly in the lower kidney area."
The left side of her face was battered, with a lump on her chin, bruising around her eye and marking around her mouth.
The victim also suffered soreness and bruising on her back, but she did not seek medical attention.
Groves told police he remembered arguing with his partner, but could not remember assaulting her or being taken to the police station. He said this was because he had been drinking alcohol that day.
Judge Dugald Matheson sentenced Groves to 160 hours' community work and nine months' supervision.
"This wasn't a pleasant situation and what's more disturbing is that young citizens of the Whanganui community are the ones that came across it and engaged police," the judge said.
"Mr Groves, your life has come unstuck in recent times, I can see that. Your plea to the assault on your partner is appropriate.
"Any repeat domestic violence offending will produce a consequence which will be custody, you know that and I know that."
