MELBOURNE, AUSTRALIA -  Five people, including two children, were found bludgeoned to death in a Sydney home in a "horrific" murder with no apparent motive, Australian police said.

Police said the victims, discovered Saturday in the Sydney suburb of North Epping, were believed to be four family members and a female relative.

"The injuries of all five people are quite definitely horrific," Superintendent Geoff Beresford told Sky News.

"They're all blunt trauma injuries to the upper body and to the heads of all the victims, which regrettably makes visual identification very difficult.

"It was an extremely violent attack carried out, I guess, with some precision."

Police said the bodies were believed to be a 45-year-old man and his wife, 43, their sons, aged 12 and nine, and the woman's 39-year-old sister.

They refused to identify the dead but media reports said two of the victims were Chinese-born Lin Min, his wife Lin Yun Li.

The deaths were initially thought to be murder-suicide, possibly linked to a domestic dispute, but Beresford on Sunday ruled out that possibility and admitted police were struggling to find a motive for the killings.

"It certainly is not a murder-suicide, it is simply, regrettably, the murder of five innocent people," he said.

"The motive is unclear at this stage, it certainly doesn't bear the hallmarks of what you might call a traditional home invasion. Robbery does not seem to be the motive (but) we haven't ruled it out."

Media reports said that the eldest male victim had witnessed an armed robbery near his newsagency last May which netted 1.2 million dollars (935,000 US).

Beresford said police did not believe the two crimes were linked although all avenues were being investigated.

He said a female member of the family was due to return to Australia from overseas Sunday and police would seek her help in the search for a motive.

Neighbour Pamela Burgess described the family as "normal".

"They're a family of five, their kids went to the local school," she told national news agency AAP.

"They were very nice, they were normal."

A police task force has been set up to investigate the murders and detectives appealed to anyone with information to contact them.