Raewyn Dailey from Napier Antiques said the thieves stole $50,000 worth of silver jewellery.
The jewellery had been sourced overseas and would be difficult to replace.
"We've put so much effort into getting our stock, and we can't just up and travel overseas and buy it again easily," she said.
Spot prices for silver had more than doubled since December last year.
Retail crime in the city had risen significantly in the past four years, she said.
"The crime is just terrible, and it's getting worse and worse, and no matter what we do security-wise, nothing seems to quite cut it any more."
The stock was especially needed for visitors coming into the city for the Napier Art Deco Festival, which was just two months away, and was a major boost to business that they counted on, Dailey said.
"You know it couldn't happened at a worse time for us. Being an antique store we're not a regular gifting shop where people come in before Christmas to buy, they actually come in when they're travelling around, and this is our busiest time, from today."
In recent years, disruptions to business from Cyclone Gabrielle and the pandemic had made an impact, especially due to the festival being cancelled over multiple years, she had earlier told RNZ.
Police are investigating the overnight break in and said they were following positive lines of inquiry.
