Ms Rusia, 29, of West Harbour, said nobody knew exactly how Genesis went missing.
"We saw him running up the road and that was the last time anyone saw him."
Panic broke out when his family realised he had disappeared and they scrambled to find him. Emergency services were called just after 3.30pm.
Someone spotted the 5-year-old out in the mudflats about 100m from where the reserve drops down to meet the water.
At a marina about 30m north of the reserve, fisherman Andy Song saw the efforts to save the boy.
"A policeman took off his jacket, pants, shoes and he just ran into the mud. It's very dangerous, you sink into it so quickly.
"He was a hero."
As the policeman tried to reach the boy, a fisherman from the marina also also tried to reach him, but he was too far away.
The police Eagle helicopter reached the scene and its crew scooped the boy out of the mud.
He was taken to ambulance staff at the reserve, where he was pronounced dead.
Ms Rusia said the family were all deeply saddened by Genesis' death, which was a total shock.