"He tried to rescue him by getting him on board his own jet-ski and he found he couldn't get him on to his jet-ski and out of the water that way.
"It was quite good waves there at that stage," Mr Stewart said.
After several attempts to hoist his brother out of the water failed, the man jumped in and attempted to rescue him that way.
"A member of the public that was on the beach came and also assisted, and they pulled his brother up onto the sand. By that stage he was unconscious," he said.
Resuscitation attempts were unsuccessful, and the man was pronounced dead by at the beach by Ambulance staff.
The father-of-three lived locally at Pines Beach, near Kaiapoi, Mr Colin said.
His name is due to be released tomorrow.
All three men were experienced jet-skiers and wearing life jackets. While surf conditions at the time were bigger than normal, weather conditions were fine, he said.