When the tremors return, residents at one Christchurch rest home band together with cups of tea and company.

Letty Valli, 86, and friend Sally Hendly, 91, say Saturday's big earthquake was frightening, but the aftershocks and not knowing when they will ease are worse.

A 5.1-magnitude quake yesterday - the "mother of all aftershocks", according to Mayor Bob Parker - have left the pair and their city on edge.

"I was crossing my fingers saying, 'Please God, let it stop'," says Mrs Hendly. "The only time you remember Him is when you need Him."

Mrs Valli: "Having each other is a big help ... I wouldn't go home on my own for anything. It makes me feel safe, being with others."

The pair, from Mayfair LifeCare in Riccarton, epitomise a city struggling to cope mentally and to pick up the pieces after the devastating quake.

Yesterday, the Treasury doubled its estimate of the earthquake's cost to $4 billion. Chief John Whitehead said the Treasury had been working on the assumption that the Earthquake Commission (EQC) was likely to pay out up to $2 billion on claims, but the wider bill would be far higher.

"If you take the effect on others beyond that, you're probably talking about something in the order of doubling that. The costs faced by the EQC, by individuals, by businesses, by insurance companies, will probably be in the order of $4 billion."

The recovery from the 7.1 magnitude quake - which has been termed a "once-in-750-year event" - was set back when yesterday's aftershock struck just before 8am, causing further damage and fraying more nerves.

In all, Canterbury has received 270 aftershocks of magnitude 3 or above. GNS Science seismologist Brian Ferris said people would have felt about 150 shocks.

Members of Canterbury's oldest farming family, the Deans, were yesterday digging up rose gardens planted a century ago, as they rushed to salvage what they could from a once-proud homestead that has become a ruin.

The massive jolt crumbled the homestead in Home Bush, kilometres from the quake's epicentre.

The family gathered yesterday in cold drizzle in front of several large piles of debris from the house.

"We're all tired. We can't even get words out right," said Louise Deans, the matron of the house.

The family's history in the district dates back to the 1840s, when they were the first to farm the Canterbury Plains. They bred Canterbury rugby greats Bob and Robbie Deans were once at the pinnacle of New Zealand farming, managing more than 26,000 sheep.

The nooks and crannies of the family home have become packed with the heirlooms and paraphernalia accumulated over more than 150 years.

Doors, paintings, ceramic beer mugs hand-made for family cricket matches, endless piles of books and loose paper, and old jars of acid batteries were among the items recovered from the cellar and attic.

Mrs Deans yesterday entered the house for the first time since the earthquake, despite the risk it could collapse.

She went straight for a china box that has been passed down through the generations.

"I had it in my mind. It was always my favourite, kept on the mantle by the fireplace," Mrs Deans said.

Everything around it was in ruins, but the box sat in the middle of the wreckage, perfectly intact.

Mrs Deans inherited it from her mother-in-law. "It was almost like saving her."

Early in the week, neighbours and a few brave builders helped to cart furniture and other large items from the house. The only item that couldn't be saved was a billiard table in the attic.

The Historic Places Trust has made particular note of the slate table's value. It was built in 1902 on the ground floor and moved to the attic in pieces.

A solution is yet to be found on how to get it out.

Mrs Deans' sister-in-law, Gillie Deans, said the soul of the family had been recovered. "It's like the soul has left the body. The building has become a shell, and the soul will live on with the family in their new house."

Back in Wellington, the Treasury said the EQC had a record $6 billion in reserves and the Government's finances were in good shape to deal with the quake's economic fallout.

Dr Whitehead said the Treasury expected the earthquake to negatively affect September quarter growth by about 0.5 per cent. Based on current Reserve Bank forecasts, that could see growth of just 0.4 per cent.

However, there was a good chance that effect would be more than reversed in the first half of next year.