Kenneth Samuela Lotu-Iiga, the son of the National MP's sister, admitted a charge of assaulting his partner in an incident on July 12.
The charge carries a maximum penalty of two years in prison. He also pleaded guilty to wilful damage in relation to the same incident.
His partner, who is pregnant, declined to comment, and the circumstances surrounding the assault are unclear.
Kenneth Lotu-Iiga - whose Mt Albert address listed on court documents is standing empty - pleaded guilty last week. He will be sentenced in November.
The revelation is the latest blow for Lotu-Iiga, whose Corrections portfolio has come under fire in recent weeks.
He faced a deluge of criticism after allegations of violence between inmates, drug use and banned items, such as cellphones, at the Serco-run Mt Eden Corrections Facility.
Corrections has since taken over the privately-run prison during investigations into the allegations.
The National MP faced further criticism this week when it was revealed at least 18 offenders on electronically monitored bail, or on electronic monitoring for sentence or release conditions, are on the run.
In the latest case, Southland man Michael Mulligan, 30, removed his electronic monitoring bracelet and has been on the run for more than two days.
Earlier this month, convicted child rapist Daniel Livingstone also removed his bracelet and went missing for more than 24 hours in Lower Hutt.
Despite criticism of his handling of these cases, Lotu-Iiga was defended by NZ First leader Winston Peters who said the National MP was qualified for his ministerial portfolio because he was Samoan.
"I think Sam has a great understanding of some of the conditions and some of the psychology of a majority of prisoners in New Zealand, who happen to be Polynesian and Maori," Peters told reporters at a party conference earlier this month.
"If you come from the biggest population group in prisons, you might have a rough idea of what some of their concerns and aspirations are. Maybe better than some other culture, that's all I'm saying."
Peters also said the MP for Maungakiekie since 2008 had not put the policies for privatisation of prisons in place, and had been left to deal with the consequences.