She was playing at the family home today when the wall, which was little more than 1m high, fell on her about 10.30am in the west Auckland suburb of Massey.
She died at the scene despite the frantic efforts of family to resuscitate her.
It appeared the incident was "purely an accident", said Inspector Lou Alofa of police northern communications.
The boundary wall was described as "very, very old" and the mortar holding the wall together appeared to be extremely dry and brittle, he said.
An autopsy on the girl's body would be carried out and her death handed over to the coroner, he said.
Glen Taylor from the St John Ambulance said family members dug her out and rushed her inside to begin emergency resuscitation as other family members called an ambulance.
As family members tried to resuscitate the toddler they were talked through the best method by the St John Ambulance call-taker during the 111 emergency call, he said.
"The family was doing CPR on our arrival. When they got there our guys continued that but the patient was obviously deceased."
