Post-mortems have been completed on all 50 victims of the Christchurch terror attacks.

In a statement, police said 12 victims have been identified to the satisfaction of the Coroner and six of those have now been returned to their families.

"Police are acutely aware of frustrations by families associated with the length of time required for the identification process following Friday's terror attack."

Investigators are also looking at improving their communications with families to ensure they are kept fully informed about what is happening.

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"While identification may seem straightforward the reality is much more complex, particularly in a situation like this," the statement said.

"Our absolute priority is to get this right and ensure that no mistakes are made.

"We are doing all we can to undertake this work as quickly as possible and return the victims to their loved ones."

Frustrations have been rising among the families of the victims as they wait for their loved ones' bodies to be returned to them.

Islamic religious law usually calls for the body to be washed and shrouded in white cloth before being buried within 24 hours of death.

However, four days since the massacre took place many are still waiting for police to finish their investigations.

Police say they are "acutely aware" of the length of time being taken to identify bodies, and are doing all they can to do the work as quickly as possible.

"Normally we shouldn't wait too long to bury, but in this case they're still taking time," said Mohammed Bilal, whose cousin Syed Areeb Ahmen was killed on Friday.

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"It's hard, especially for Syed's family in Pakistan. He was the only son. It's really hard for the parents.

"We have to wait. It's the Government process. They have to [accept that]."

He said the burial rituals have stood since the birth of the faith.

"That's the process we all have to follow, every Muslim has to follow. It's been like that for 1400 years," Bilal told the Herald.