No patrol cars were involved in the crash, a police spokesman said.
A Fire Service spokeswoman said three cars were involved in the collision and three people were trapped in one of the vehicles and had to be freed by firefighters.
An employee at the nearby Caltex said he saw a car speeding along Great South Rd, heading north.
An undercover police car was on the scene within 10 seconds, the employee, Dilpreet Singh, said.
"I heard the noise it was very loud. When it collided, there was a car, an undercover cop, heading to the north."
Agnes Morunga, who works at the nearby Carl's Jr fast food restaurant said several police cars were on the scene.
"We can see all the lights flashing."
The lights at the intersection went out after the crash, Morunga said.
Police said motorists should avoid the area because parts of the road would be closed.