He also admitted in court to smoking cannabis earlier that day.
Mrs Cooper's family said they were angry he was not jailed for his actions.
"We were hoping for a custodial sentence with the seriousness of the crime," her brother Aiden Wards told Radio New Zealand.
"And we didn't know until court that he had 64 previous [driving] infringements and lost his licence four or five times.
"We just thought, 'he's not learning from his mistakes and he's kept going until he's killed someone', and then he gets six months home detention as a slap on the wrist."
The judge gave Peacock credit for taking part in a restorative justice session with his victim's family, showing remorse, having a young family and a stable job, Mr Wards said.
"But we were pretty shocked [by the sentence]."
He was "glad" to have taken part in the restorative justice meeting, "because we got to confront him", he said, but would not have agreed to it if he had known it would influence Peacock's sentence.
"We were told by the restorative justice and the police that it has no effect on the sentencing, but Judge Roberts said he would take two months jail off the sentencing because of that. He's taken it into account, but he took other things as well.
"I got a lot out of the restorative justice, but ..."
He also did not think Peacock was remorseful.
"I think he was worried about going to prison, but I don't think he showed a lot of remorse in a lot of his actions."
Mr Wards said the family had found the judicial system "very disappointing and frustrating", and had been left feeling "powerless".
Peacock was also disqualified from driving for two-and-a-half years and ordered to pay $6000 in emotional harm reparations to Mrs Cooper's family, Radio New Zealand said.