A woman was stabbed in the stomach and neck during a domestic incident in Paeroa on Thursday morning.
Police say the woman, 53, intervened after her niece was attacked.
She is in Waikato Hospital for surgery later in the day.
Police say the niece has bruises on her body and defensive wounds to her hands caused by a knife, as well as a cut to her neck.
Armed police responded to emergency calls to the house and arrested a man, 25, at 7.40am.