Mr Mills' incomplete remains were found in the garage of an abandoned property on Barrack Rd, Mt Wellington, last September. They were confirmed as his this month.
Cooper was charged and police arrived at his home, two doors from where he allegedly left the remains, to do forensic testing.
Police believe Mr Mills died at Cooper's house after an "altercation". They say they cannot reveal a cause of death as parts of his body, including his skull, are yet to be found.
In court this week, lawyer Mina Wharepouri said Cooper told police what he did with the missing "material", and where they could find it.
It is understood the search area now spans between Auckland and Rotorua, where Mr Mills was reportedly last seen alive.
Cooper had said the death was a "tragic accident" after he tried to protect himself from a violent attack.
A source close to the investigation said detectives spent much of this week taking statements from "dozens" of people who knew both men in Rotorua.
The source said Cooper, nicknamed Zero, took Mr Mills there to meet his "goth" mates. They went from house to house as Mr Mills made new friends and impressed them with his guitar playing.
Cooper picked him up from a friend's house in July to return to Auckland. When details of the case emerged this week, that mate realised he may have been the last person to see Mr Mills alive and told police.
Police say Cooper took Mr Mills' remains to the Barrack Rd garage sometime last year.
They also say he went to great lengths to make people think Mr Mills was alive.
On December 22 in 2009 a message supposedly from Mr Mills was posted on family Bebo pages.
Their concerns about Mr Mills quashed, they enjoyed Christmas together without him.
But he was already dead.
