The four-year criminal investigation into the deadly collapse of the Canterbury Television building in the February 2011 earthquake is in its final stages, police say, with a decision on whether charges will be laid expected within weeks.

Investigators have interviewed more than 100 witnesses and raided the offices of Engenium Consulting Engineers, formerly Alan Reay Consultants, which designed the ill-fated Christchurch office block in the mid-1980s.

Engineers have made replicas of the structure to test its critical elements.

Now, legal reviews have been completed by Crown Law and the Christchurch Crown Solicitor.

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The decision on any prosecution will be made after careful consideration of the file and the two legal reviews, police said today. It is expected to take several weeks to complete.

"As we have previously stated this has been a very complex, technical investigation involving a range of expert advice to consider," said Detective Superintendent Peter Read.

"The decision will be ultimately made in accordance with the Solicitor General's prosecution guidelines, which determines every prosecution decision by police.

"While police are unable to pre-empt what the decision will be at this time, we can say that the issues before us are finely balanced.

"We are acutely aware of the length of time this is taking, and the impact this is having on the victim's families. We continue to be very appreciative of their patience.

"Once a decision is made, families will be told before it is made public."

The six-storey concrete CTV Building pancaked in the magnitude 6.3 February 22, 2011 quake, claiming 115 lives.

In 2012, the royal commission of inquiry found serious errors by engineers, structural designers and the Christchurch City Council.

After the report's release, families and friends of the victims called for legal action to be taken against those responsible.

The commission said Dr Alan Reay should have recognised that his employee David Harding was working beyond his limits when designing the building in 1986.