"I reached for my phone and was screaming," Mr Mistry said.
"They ran out and I chased after them, calling 111. I was bleeding and I couldn't talk but a customer came over and took my phone to tell them what had happened. It was so scary. I think they would have killed me. They hit really hard and aggressively."
His wife Kamini got a phone call telling her to go back to the shop because of the attack.
"The blood was running all down his face ... I was shaking and crying.
"We've had lots of customer support - everyone wants to help and we're very grateful, especially for the man from the Mobil and our neighbour who helped straight away. A man also prayed for us outside the shop."
Police said the offenders were described as teenagers, one wearing a grey and maroon hoodie, a red bandanna on his face, black cap and a black backpack on his back. The other was wearing a maroon hoodie, blue shorts, black bandanna and a black bag across his shoulder.
Anyone with information is asked to contact Rotorua police on (07) 348 0099, or Crimestoppers on 0800 555111.