Toddler Nia Glassie had a blood clot in her head that was 10 to 14 days old when she died, a pathologist told a court today.

Dr Simon Stables said the clot covered a large area about 9cm to 10cm in diameter between Nia's scalp and brain, and was likely due to trauma.

Blows to the head and falls from a height were usual causes of such injuries, he told the High Court at Rotorua.

Such injuries were also common in boxing when the boxer received a blow to the head and fell to the floor.

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Dr Stables performed an autopsy on Nia on August 4 last year, the day after she died.

She was admitted to hospital in a coma 12 days before her death, and it is alleged that brothers Wiremu and Michael Curtis murdered the 3-year-old by kicking her in the head two days earlier.

Dr Stables said Nia's injury was caused by "a substantial degree of force", and possibly a kick to the head.

He said the clot in Nia's head was the result of veins rupturing, leading her brain to swell.

The post-mortem examination had shown a large part of the right side of her brain and some of the left side was dead by the time her life support was turned off.

This cerebral infarction - or brain death - caused Nia's death, Dr Stables said.

He ruled out a brain infection, acute encephalitis, which had been suggested as a possibility when Nia was first taken to hospital.

He said the 3-year-old was 96cm tall and weighed 11.5kg, which was thin for her age and height.

An external examination of her body showed multiple scars, including a 6cm one on her abdomen and healing scabs on her legs, but Dr Stables said there was nothing external that pointed to the cause of death.