"It was possibly caused by a medical event."
A St John spokesperson said a patient was taken by ambulance to Hawke's Bay Hospital in a serious condition.
Cordons were put in place at about 11.30am and nearby businesses and houses were evacuated until cordons were lifted at about 1pm.
The main was sprayed with water to disperse the gas before a gas company arrived and isolated the leak and fixed it, Mr Nichol said.
A nearby resident, who did not want to be named, said he heard a loud bang and was then evacuated from his house.
"We could smell the gas from our house."
A police spokesperson said the gas leak was capped just before 1pm and cordons were lifted.
"Police would like to thank those in the area for their patience and cooperation as we dealt with the incident."