She died a short time later.
Today, Van Hemert admitted a charge of murder at the High Court in Christchurch.
The court heard for the first time the circumstances behind Te Pania's brutal murder.
Over Christmas last year, Van Hemert found out his ex-partner had a new man, the court heard.
She noticed that Van Hemert's mental health started to deteriorate and authorities were brought in on December 30.
It was concluded that his mental health could be managed with medication – and he was left to sleep on the basis that family would admit Van Hemert to Hillmorton mental health facility the next day – December 31.
However, during the night he took off, and around 3.47am was seen driving a Mazda ute on Manchester St – the home of the city's street workers.
He stole two number plates from other parked vehicles to hide his real registration.
Te Pania, who was working as a prostitute between Aberdeen and Salisbury streets, was picked up by Van Hemert.
They stopped in Burnside to talk money and services, and it was there than Van Hemert claims she got angry and lashed out.
Van Hemert then pulled out a large knife similar to a fish filleting knife and stabbed Te Pania multiple times.
He then bashed her over the head several times with a rock.
CCTV later showed him driving erratically through the city and at about 6.45am he drove into the secure Air NZ engineering site on Orchard Rd where airline staff called police.
Officers found the victim in the front passenger seat. She would die soon after with extensive injuries, including knife wounds to her leg, face, abdomen and throat.
When Van Hemert was interviewed later that morning by police, he alleged there had been altercation with Te Pania and that she attacked him.
He said they both "saw red" and that he had "sliced and diced" her and murdered her before freaking out and taking off.
Van Hemert had originally pleaded not guilty to murder, but entered a guilty plea this morning.
Justice Cameron Mander remanded Van Hemert in custody to be sentenced on December 4.
