A legal battle to save Christ Church Cathedral is set to rage on after high-profile Auckland lawyer Mai Chen claimed the demolition order may be unlawful.

Campaign group Restore Christchurch Cathedral sought advice from Chen's firm ChenPalmer to back up action taken by the Great Christchurch Buildings Trust (GCBT), which this month sought a binding court ruling on whether the Anglican church's deconstruction plans breached an act of Parliament protecting church buildings.

Chen's view was the New Zealand Historic Places Trust (NZHPT) breached its statutory obligation to protect heritage buildings by approving the Canterbury Earthquake Recovery Authority's section 38 demolition order.

In her opinion, the NZHPT signoff was required under the Canterbury Earthquake Recovery (CER) Act.

The NZHPT was obligated under the the Historic Places (HP) Act to consider the "least possible alteration or loss" of heritage values despite the act being amended for emergency demolition after the February 2011 quake, Chen said.

Expert engineering advice proved the cathedral could be safely maintained in its existing state or restored to pre-quake condition.

"The purpose of the HP Act clearly requires that the cathedral, as a heritage building and an archaeological site, must be protected and preserved," she said.

"This purpose is subject to the purposes of the CER Act to the extent of inconsistencies, but in our view there are no such inconsistencies.."

Whether the building owner wanted the cathedral demolished was "irrelevant", Chen said.

The cathedral was an "essential and irreplaceable" part of Christchurch and New Zealand's cultural and historic heritage.

"The HP Act requires that it be protected, at least in circumstances where demolition is neither necessary nor inevitable. Our clients would be highly concerned by any decision to authorise its demolition in these circumstances, and would seek to challenge the lawfulness of that decision," she said.

Restore Christchurch Cathedral spokesman Mark Belton said it was now "crystal clear" there was no justification for the cathedral to be demolished.

It was likely the group would seek a judicial review if the GCBT case failed.

A Cera spokeswoman said the NZHPT and Christchurch City Council were consulted before section 38 notices for heritage-listed 1 and 2 buildings were issued, but approval was not required.

A NZHPT spokeswoman said the trust received the letter yesterday and needed to "work through the points raised".

The Press understands that approval for demolition of the spire only had been given and the NZHPT board had passed a resolution in April endorsing the call for a halt to further demolition.

- © Fairfax NZ News