Nine Christchurch homes damaged in Friday's giant gas explosion have been declared off-limits and WorkSafe has taken over from police as the lead agency investigating the blast.

Neighbours and passersby who pulled stunned survivors from the burning debris were amazed that nobody was killed instantly in Friday morning's blast.

While investigations into the explosion's cause continue, one man remains in a critical condition at Christchurch Hospital. Another is in a stable condition, and a Counties Manukau DHB spokeswoman said a patient at Middlemore Hospital, which has a serious burns unit, is also stable. Three others have since been discharged.

The 10.14am explosion obliterated a Marble Court house in the northern suburb of Northwood. Blast waves scattered chunks of roof tiles and other debris around the neighbourhood.

Several neighbouring houses have major damage, including blasted walls, caved-in garage doors, smashed windows and eaves.

This afternoon, Christchurch City Council's head of regulatory compliance Tracey Weston confirmed that four Dangerous Building notices have been issued, which prevent "use or occupation" of the properties.

And another five houses have been issued Restricted Access notices, which only allows specific people like insurance agents, structural engineers, specialists, builders or tradespeople, and owners "for retrieval of personal items" in the building.

A gas contractor who worked on the Marble Court house, which was having gas issues, just one day before it exploded, arrived at the chaotic scene on Friday and fronted to police officers.

Canterbury Metro Area Commander Superintendent Lane Todd referred inquiries to WorkSafe, which is now acting as lead agency.

Some homeowners face a long wait before they can return to their homes.

Cordons remain around the most-affected properties.

Indraj and Akita Fonsaka who live across the road said on the weekend that friends and family were looking after them as they said their insurance refused to stump up for temporary accommodation.

Their lives have been thrown into turmoil and, like many of their neighbours, they are unsure when, if ever, they'll be allowed home.

"We were living very peacefully here," said Akita.

"We have lovely neighbours, it's just a nice quiet street."