Freedom Taikato died in a suspected homicide on New Year's Day.
He died of his wounds after being stabbed by a former convict at a charity event.
They will appear in court on assault charges.
Det Insp Scott Beard had previously been praised by the Millane family.
Many wore white ribbons, a symbol representing opposition to violence against women.
The family will hold a private burial to remember her.
The 22-year-old was found dead on New Year's Day.
The 25yo died earlier this month after being dropped off at Christchurch Hospital with serious injuries.
His bid at the Court of Appeal failed in October.
Charlene Kraatskow says the family's now focused on "healing" - but says they "won't just give up" on justice.
What happened to Grace Millane was rare, but what enabled it is all too common.
Brian Hilton's 2016 death was thought to be an accident until new information came to light.
More than 100,000 people have signed a petition calling for a harsher sentence.
She was given less than a year of home detention for killing a teen in a hit-and-run.
They also praised New Zealand's "heartbroken" response to the 22-year-old's murder.
But the Prime Minister hasn't got it yet.
The event follows vigils held across the country on Wednesday .
Shannon Baker was found dead on December 11.