He was speaking to police but his foreman explained what the man had seen.
The foreman said the worker heard a woman screaming and when he looked up he could see what he thought was a woman being stabbed in the back.
She then fell to the ground.
The worker ran to her and before he got there another person in a car had stopped.
When the worker got to the woman, she was already dead.
It is understood a number of people who saw the incident unfold are at the Henderson police station giving formal statements.
They are said to be traumatised by what they saw this morning.
Police confirmed a man was now in custody.
"Police apprehended a man a short distance from the scene, who is now assisting us with our enquiries," said a spokesperson.
"Police are not seeking anyone else in connection with this incident."
The woman's body remains lying on a grass area at the side of the road, covered by a sheet.
A forensic scene examination is underway and the area has been cordoned off.
Police will not be drawn on further details of the killing.
They will not say where they located the man believed to be behind the stabbing or if he was alone when he was apprehended.
"At the present time we have nothing further to add to our statement from earlier this morning, but we will look to provide any further updates proactively as information becomes available, including around the matters you ask about below, if possible," a spokeswoman said.
* Police ask anyone who may have witnessed the incident or who has information that can assist to call Waitemata Police on 09 839 0697 or Crimestoppers anonymously on 0800 555 111.