A murder investigation was underway and officers were working at several crime scenes to secure evidence and establish what happened, Dick said.
The investigation would also look into how the suspect, a 23-year-old man, was able to get a gun into the police station.
The Independent Office for Police Conduct said the firearm used was a non-police issue and a revolver had been recovered from the scene.
Ratana went to school at Palmerston North Boys' High School before attending the University of Otago where he played for Ravensbourne RFC.
Recently, Ratana had been a coach at East Grinstead Rugby Club where friends described him as "inspirational" and "larger than life".
"He helped out at the club a lot. We lost funding and he came in and helped us," Ratana's friend, Paul, told Sky News.
Metropolitan Police collect floral tributes at Croydon Custody Centre following the fatal shooting of Matt Ratana. Photo / Getty Images
"He was inspirational - so vibrant, so bubbly, larger than life. He will be a huge loss to the club.
"The man was a machine. He was at training last night from 7 to 9pm. That's the last time I saw him. He was due to retire soon and was looking forward to it."
Police stations across the UK are flying their flags at half-mast and officers also held a minute's silence to honour Ratana.
Our flags at Force HQ and City of Manchester Divisional HQ fly at half mast as a mark of respect for our fallen colleague in #Londonpic.twitter.com/Q4HHx9reyc
He was posted to Charing Cross where he worked as a constable on the streets of the West End and Westminster in various roles.
Later, he worked with the Territorial Support Group and in Hillingdon.
In 2010, he worked as a sergeant in Hackney in the response team and in neighbourhoods, Dick said.
"Five years later, in 2015, he moved to Croydon, where he worked in response, in neighbourhoods, and then our detention command," she said.
"In all, nearly 30 years spent as an uniformed officer serving the public of London."
RIP Sgt Matiu Ratana, 54. Shot dead in the line of duty. New Zealand-born Sgt Ratana worked in @metpoliceuk for nearly 30yrs & was just two months away from retirement. An absolute tragedy. Thank you for your service Matt. RIP. pic.twitter.com/P7VazyVjyd
The suspect was arrested by regular patrolling officers for possession of ammunition and possession of class B drugs with intent to supply in Pollards Hill after a stop and search.
"We are a large, professional, resilient, and very experienced police service.
"As we mourn the loss of a much-loved colleague, senselessly killed, be under no illusion that our resolve to protect the public and to tackle violent crime – whoever may be responsible for it – will be undiminished."
The Independent Office for Police Conduct says the suspect was handcuffed to the rear after he was arrested.
IPOC regional director Sal Naseem said he was transported to Croydon Custody Suite in a police vehicle, then escorted into the building.
"He remained handcuffed to the rear and seated in a holding area in the custody suite," Naseem said.
"His handcuffs remained in place while officers prepared to search him using a metal detector.
