"Normally we do everything so carefully, so we don't know why it happened, but we are very upset," she said.
The pair had previously owned a bakery in Auckland, after immigrating to New Zealand from Cambodia about 25 years ago.
They have owned the bakery, along with the five other businesses on that stretch of Heretaunga St, for the past 12 years, but the food outlet is their only source of income.
The woman said everything, including large appliances would need to be replaced, having disintegrated in the heat.
"I don't know how to put it into words - everything is gone," she said. "We can't close for too long because it is no good for us."
Taj Spice owner Buddhi Bhandari, whose business neighbours the bakery, said his business would be closed for between two to three weeks, while everything was checked and replaced.
Although the worst of it was confined to the bakery, his food storage area had sustained smoke and water damage.
Bhandari expected it would cost him up to $15,000, not including lost earnings.
He said it has hit his family's business "very hard" and was worried about customers moving elsewhere.
Fire and Emergency specialist investigator Murray Kidd said the fire was not suspicious, but there was "too much" damage to determine exactly what the cause was.
"It would appear that something has either failed or been left turned on, or something of that nature."
Kidd said he was certain it had started in the back of the building which housed the bakery and prep areas.
"We're still looking at it. The insurance people are investigating, so they may come up with some other ideas."
He urged people to take that "one step further" to ensure things like this don't happen, including having mechanisms in place that switch off after a period of time.
