More than 100 people gathered in Christchurch this afternoon, to protest the police's decision not to prosecute the designers of the Canterbury Television building, which collapsed in the February 2011 earthquake.

Photo: RNZ / Maja Burry

The six-storey office block was flattened in seconds during the 6.3 magnitude earthquake, killing 115 people.

Police had initially planned to lay criminal charges, but 10 days ago they announced that the evidence was not sufficient to provide a reasonable prospect of conviction.

They were guided by the advice of the Deputy Solicitor-General, Brendan Horsley, who said the chances of a successful prosecution against building designers Alan Reay and David Harding were low.

Photo: RNZ / Maja Burry

A law which means no one can be held criminally responsible for a death that occurs more than a year and day after the contributing crime was a key factor in the decision not to prosecute.

During a peaceful protest in Christchurch today Julie Hibbs, whose mother died in the disaster, said the law needed to be repealed.

"They need to change the law plain and clear, we just want justice and someone should be held accountable for that building."

Ms Hibbs said the families had waited too long for justice.

Sonya Rockhouse, whose son Ben Rockhouse was one of the men who died in the 2010 Pike River Mine disaster, attended the protest today to support the CTV families.

She said New Zealanders were fed up with nobody being held accountable for workplace disasters.

"They're hearing the same rhetoric that we heard, not enough evidence to prosecute and not in the public interest and it's just a load of rubbish, you can't just be responsible for the deaths of people and then just walk away from it - it's wrong."

Sonya Rockhouse said she could relate to CTV families' struggles.

One of the protest organisers, Maan Alkaisi, who lost his wife in the building collapse, said the families wanted justice and accountability.

"We are asking for a fair trial with a jury to reach a just decision," he said.