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The witness said a man had run up to the woman, stabbed her multiple times and then ran off and fled in a nearby vehicle in the suburb of Massey.
The witness told the New Zealand Herald they tried to give the woman CPR but she died at the scene.
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."
Officers were called to the scene after a report of an assault around 8.35am this morning.
Multiple cordons are in place on Westgate Drive with diversions in place, and will be for some time.
The witness is currently being spoken to by police.
Police have cordoned off a reserve area at the junction of Westgate Drive and Bonny Crescent.
St John ambulance was called at 8.36am to Westgate Drive but referred media to police for other details.