A one-year-old girl has been found dead strapped into a hot car outside a Sydney childcare centre in a “most traumatic event”.

The child was left inside the vehicle for “an extended period of time on a very hot day” in Earlwood, in Sydney’s inner-west, police said.

She was reportedly discovered when a male relative arrived to collect the girl, but was told by childcare staff that she had not been dropped off.

The man then ran outside and discovered the child inside the vehicle — strapped into a booster capsule, Sunrise reported on Wednesday morning (AEDT).

Temperatures in Sydney were in the 30s on Tuesday amid a NSW heatwave warning.

A passerby contacted emergency services about 5.35pm Tuesday to report a child unresponsive in a vehicle on Marana Road, Earlwood.

NSW Ambulance paramedics arrived, but the toddler could not be saved. She was declared deceased at the scene.

Officers from Burwood Police Area Command established a crime scene and started an investigation.

A male “occupant” of the vehicle — who was related to the child — was assisting with inquiries at Burwood Police Station.

He was taken to hospital suffering from shock.

Police tape off the street outside the childcare centre in Earlwood. Photo: 7NEWS

Superintendent Christine McDonald said the child’s death was “an absolute tragedy”.

“The information provided to us suggests that this baby was left inside the vehicle for an extended period of time on a very hot day,” McDonald said at the scene.

“Leaving a child in a vehicle at any time can be dangerous.

“Leaving a child inside a vehicle for an extended period of time can be deadly.

“This is an absolute tragedy — the death of a one-year-old baby girl.”

McDonald confirmed that the male was a direct relative of the baby.

“He is suffering shock and he is currently being assessed. Once he is able to we will speak to him to obtain more details,” she said.

The child’s mother is also assisting police with inquiries.

McDonald said her thoughts were with all those involved and impacted by the “deeply traumatic” incident.

“This has a rippling effect right throughout our community because so many of us can put ourselves in a similar situation.”