The witness says he decided to speak out after another motorist - travelling in the northbound lane - claimed the officer was changing lanes after getting stuck behind traffic.
"He switched three lanes at high speed and there was something of a bend on the motorway and he lost control of his car. It went airborne, it was unbelievable. I was - like anybody who must have saw it - was white with shock," the other motorist said.
However, the Auckland man said cop car never changed lanes.
He said he was travelling in the right-hand, or fast lane, of the southern motorway when he saw a police car coming up behind him with its lights and sirens on.
He dutifully pulled into the middle lane and let the car pass, before noticing a second patrol car coming up the fast lane.
"As he went past me, I was going 100km/h, he must have been absolutely motoring it because he just flew past me.
"He wasn't changing lanes at all. I watched him come up behind me and race past and I was literally thinking to myself 'he is going way too fast given the conditions because obviously it was a bit greasy on the road with rain and stuff overnight.
"There was a slight bend, just after he passed me, about 200m or 300m up the road ... and as he went round the slight bend he just literally lost control, his nose went straight into the barrier and then ... he slid so quickly across all three lanes, hit the barrier and it went airborne and flipped in the air and landed on the roof."
The man said he was surprised, but pleased, to hear the officer was able to walk away from the crash, given how "horrific" it looked.
"I have nothing against the police but it's literally a case of him going far too quick.
"The thing that annoyed me though, yeah I appreciate that you're on a priority call but you were the second police car, not the first, so you didn't need to be going that fast in general it's just stupid, given the conditions.
"That's why the crash was so big, because he was going so fast."
Given the crash was at 7.30am during the holidays there were few cars on the road, which was fortunate, he said.
The man said he kept his eye on the patrol car as it "ripped past" and as soon as he saw the car's wheels slide out, he started pumping his own brakes as he wasn't sure where the car would end up.
"I knew he was going to all over the shop ... it wasn't a big corner or anything but it was just enough for his wheels, given the speed he was going on the wet motorway, that he managed to slide out."
When contacted, a police spokeswoman said at the time, the officer was responding to a priority job - an attempted robbery in South Auckland.
However, they couldn't comment about the man's version of events.
"The Serious Crash Unit is investigating the circumstances of the crash and as such it is too early to provide any other details."
