Inquiries into Serenity's death took months as detectives pored over evidence and called in overseas forensic pathologists to assist after "areas requiring further investigation" were identified.
"To get to the point where we have sufficient evidence to bring this matter before the courts highlights the hard work and long hours put in by an investigation team who always remained focused on doing what is right for a little girl, whose life ended so tragically," said Mr Greene.
The arrest was testament to the Ngaruawahia community, who would not tolerate violence to children and had rallied behind police, he said.
"It also vindicates the very strong relationship Waikato police have with Child, Youth and Family, whose assistance in this case, from start to finish, has been invaluable."
Serenity, who had just learned to crawl, was found lying in her room with eyes half-opened at the Havelock Rd home she shared with her mother, uncle Drew Scott and Ellery.
She had severe brain injuries, a broken rib cage and genital injuries.
Two days later, on April 28, family members surrounded her at the Starship in Auckland as her life support was switched off.
Last night, Ellery's Whatawhata grandfather - who helped to raise him and told the Herald last year that his grandson was innocent - said he was in shock and could not believe what had happened.
"I probably will go along to court but we don't know what's happening ourselves at the moment. We are very shocked," said Mr Ellery, who has cancer.
Cherie Kurarangi-Sweeney, a former neighbour of Ms Scott, said she was pleased to hear of the arrest.
Ms Sweeney, who started Nark (Nation of Advocates for the Rights of Kids) as a result of Serenity's death, was at her Ngaruawahia home on the day the infant was found injured.
She could hear Ms Scott arguing with her brother Drew, and lots of screaming.
"We would like to congratulate Waikato police for securing the evidence to make the arrest. It's fair to say that we have been waiting for this day for a very long time."
On January 6, Ms Scott gave birth to a child fathered by Ellery. Child, Youth and Family took the baby less than 48 hours later.
Ms Scott's 6-year-old son, Travis, has been in CYF care since the incident last year.
Ellery appeared with Ms Scott on Campbell Live last year to protest his innocence and to rebut claims that he had anger issues.
He said in the interview that he had tried to do CPR on Serenity. "It was too hard and it wasn't working. Chelsea was a mess and I was shaking."
Ms Scott was asked if she believed that Ellery was not responsible for Serenity's death. She said: "Yes, I believe him. People haven't seen the way Matthew is with kids."
Ms Scott said she knew who the baby's killer was and pointed the finger elsewhere. On Facebook earlier, she had said the killer was walking around "as if nothing had happened" and she wanted to see justice done. "I know for a fact who killed her."
