Oamaru teenager Aaron Macahan with his mother Jackie in Dunedin Hospital yesterday. Photo by Peter McIntosh.

The parents of a boy who was sent home from Oamaru Hospital and told to "harden up" were later told by doctors their son had a split kidney and would have died if he had been left untreated.

Mark Macahan said his 14-year-old son Aaron was taken by ambulance to Oamaru Hospital with a swollen hand and complaining of a sore back, after he fell off his bike, last Saturday.

A doctor in the emergency department looked at Aaron's back and said it was a soft-tissue injury. Mr Macahan said he was told his son needed to "harden up".

He and his wife took Aaron home, but he vomited and urinated what appeared to be blood a few hours later.

However, as he had been drinking grape juice earlier they were not sure if it was blood, so Mr Macahan got a urine sample container from the hospital.

When Aaron urinated blood again about 11pm, his father took him and the sample back to hospital.

There was "a bit of a panic" and Aaron was sent to Dunedin Hospital in an ambulance, Mr Macahan said. A CT scan showed he had split his kidney and was bleeding into his back.

"Doctors [in Dunedin] said if we had left him he would have died."

He spent two days in the high dependency unit before being transferred to ward 4C and is expected to be allowed home on Wednesday.

"The wife and I were pretty angry about it. At the least there should have been a urine sample or something done [at Oamaru Hospital]."

Hospital manager Robert Gonzales said privacy issues made it inappropriate to respond in detail through the media and he urged the family to make a formal complaint and tell the hospital their story first-hand.

Mr Macahan said they would be making a formal complaint.

Aaron's kidney is still working and is expected to heal itself without surgery.