"When you see somebody for eight to 10 hours a day, you really get to know how they think ... and how they feel about things.
"We're going through an emotional rollercoaster at the moment."
Mr Mulu said he and Mr Fungavaka often shared stories of their children, especially as they had two daughters around the same age.
"We would always ... compare stories about being a better dad.
He told how he and Mr Fungavaka, who was in Tonga for his grandfather's funeral, first met when he joined the Mangere East policing team three years ago.
"It was rough for him growing up in Mangere. He wanted to make a difference for people in the area he grew up in."
Counties Manukau District Commander superintendent John Tims said New Zealand police had been notified three people had been arrested in relation to Mr Fungavaka's assault in Tonga.
"The NZ police and myself are unable to comment. The Tongan police have the lead on the investigation."