Redfern police were called to Wyndham Street about 9.15pm on Friday night after reports a group of men had gained access to a building site.

Mojo, a German shepherd only two years and 11 months old, was searching for the men with his handler when he fell and suffered extensive injuries. Mojo's handler rushed him to the Sydney University Emergency Animal Clinic where he died soon after.

"General purpose police dogs are trained primarily to track or search in order to locate offenders, missing persons or property," said Superintendent Donna Adney. "Unfortunately, their job is quite dangerous and involves inherent risks.

"The handler and his family are greatly saddened by Police Dog Mojo's premature passing. NSW Police dogs live with their handlers and as such form a very close relationship. To a police dog handler, their dog is their companion, partner, friend, workmate, playmate and a member of the family. It is an extraordinarily sad time for the NSW Police Dog Unit and the loss of Mojo will be felt by all our officers."

Dog trainer and owner of the K9 centre Martin Dominic said German shepherds are a popular breed for policy and military work because of their intelligence.

"They can be trained in a wider range of exercises and from a younger age than other breeds," he said. "They are very adaptable to police work and make a good dog to have around under threat if they're trained correctly.

"They are a good partner, a social animal and when they bond well, they will put themselves in harm's way for the handler."

Mr Dominic said his thoughts were with Mojo's handler: "It would affect them like they're losing a human friend; there's a lot of mourning and grieving".

Tributes have begun on the Police Dog Unit's Facebook page, with many comments on a post about his death expressing condolences for the squad and Mojo's handler.

"How sad to read of this," Corina Robinson wrote. "No one really understands the value of working dogs more than their handlers who are also best friends. A huge loss to you all and sadness to all dog lovers."

Erin Child wrote: "Thanks Mojo for helping keep the public safe. RIP."

Mojo was trained by the Police Dog Unit in 2012 and had only been in active service for 14 months. 

Police have continued investigations into the break-in at the building site.