One person is in a critical condition, and two in a serious condition, St John says.

Race organiser say they can "confirm at this point that during the Auckland Marathon today, six participants have been transported to a nearby hospital where they are receiving medical treatment. We have no further details at this time on the six (unrelated) incidents and will continue to offer the athletes our support."

A half-marathon competitor said she saw three people collapse during her run.

"I saw people lying on the ground being helped by medics at 3 different spots on the track.

"It looked like one woman had collapsed just past the highest point of the harbour bridge - she was being helped onto a stretcher by St John ambulance staff," she said.

"Another man, who looked quite young, was being stretchered away from the track just 3km from the finish line. He was as white as a ghost."

One person collapsed on the harbour bridge.

An image on social media appears to show a marathon runner collapsed on the sidewalk in the viaduct basin on Customs Street West.

Auckland Marathon organisers have been asked for comment.

More to follow.