The crash happened near Gayhurst Bridge in Christchurch. The bridge is expected to remain closed on Thursday morning.

A woman who was one of the first to assist a pedestrian found lying critically injured in the middle road says he had "clearly been hit".

Helen Jones-Orft was one of the first to the scene on Gloucester St, at the intersection with the Gayhurst Bridge, in Avonside, Christchurch, while driving home from work around 2.45am on Thursday.

She said she saw a man in a blue jacket who had "clearly been hit" on the road and another witness called emergency services. She parked on the bridge to assist arriving paramedics trying to locate the man and waited for police.

Two dogs running around the man huddled "right there with [him]" as he fought for his life and kept "nudging" him until they were held by another witness, she said.

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"It looked worse than anyone should see … so sad."

A police spokeswoman said emergency services were called to a report of a man being found but no-one had come forward to say they witnessed the incident. Initial reports suggested the man had been hit by a car, she said. He was taken to Christchurch Hospital in a critical condition.

It had not yet been confirmed how the man was injured and an investigation was under way, she said. No further details of the incident were available and no-one else had been spoken to yet.

Jones-Orft said she did not think the early morning darkness would have obstructed driver's vision. The road was relatively clear down the bridge to the first bend on Gayhurst Rd but "not as lit as town", she said.

"It shouldn't have happened."

Christchurch Transport Operations Centre spokesman Luke Thomas said Gayhurst Bridge, a crucial city east link, was closed during the morning peak causing some heavy flows on the alternative routes.

The road reopened at 10am. There were also reports of ice affecting roads in the Waimakariri District so road users were asked to take extra care and watch their driving speeds.

A pedestrian was killed after being hit by a ute and thrown from a North Canterbury bridge into a riverbed in May.

Emergency services were attended the crash on the Depot Rd (State Highway 72) bridge over the Eyre River near Oxford, north of Christchurch, when a Cantabrian man aged in his mid-50s stepped onto the road into the path of an oncoming ute on the single-lane bridge.