The alarm was deafeningly loud, and the robber had panicked and fled.
He "looked like he was too scared", Mr Jasmat said, and had fled the dairy on foot, taking off first in one direction and then changing his mind and heading in the direction of Worksop Rd.
The incident had happened in a matter of seconds.
He had not really registered what kind of weapon the man was holding.
He had just seen the robber had something in his hand, Mr Jasmat said.
"He came in and panicked and then he was gone."
The robbers's face was covered by the balaclava, and all he could see was his eyes, he said.
His wife, Kamla Jasmat, said she and her husband had run the store since 1993 and worked 14-hour days, seven days a week.
It made her angry that people thought they could come in and steal their property, she said.
"It's our property and I don't want anyone to scare us.
"I won't let anybody run me out of here because I belong here.
"We don't put up with any nonsense.
"We work hard for our money."
The would-be robber was described as wearing a balaclava with a camouflage pattern, a grey top, black track pants and tan-coloured shoes with white soles.
Wairarapa police said they were investigating the incident.
They have asked for anyone with information to contact Wairarapa police on 06 370 0300 or alternatively call Crime Stoppers anonymously on 0800 555.