A 30-year-old woman from Manurewa in south Auckland has been arrested for allegedly assaulting Hamilton woman Rae Portman before she went missing in June.
Detective Inspector Mark Gutry says the arrest is not related to Ms Portman's disappearance.
"The investigation team have been investigating Rae's movements in the days leading up to her disappearance," he said on Tuesday.
The woman is due to appear in Manukau District Court on August 20, charged with assaulting Ms Portman in Manurewa on June 8.
Ms Portman, who police say is involved with the drug scene, was last seen on June 20 when she booked into a Mt Albert motel and travelled to east and south Auckland.
Police have said they have grave fears for her and have set up a hotline (0800 723 767) and a Facebook page for people to offer information.
NZN