A mother and her seven-week-old baby were on a petrol station forecourt when an armed robber pulled a gun at the counter.

The pair were at the Torbay Mobil on Auckland's North Shore when the armed bandit demanded money.

"I was still in my car so I hopped in and was [about] to drive off when I realised what was happening."

The woman posted on a local social media group that a "brave young guy" pulled up behind her on the forecourt and chases the robber down Deep Creek Rd.

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"Don't know who the young guy was but good on you and thank you," she wrote.

A police spokesman this evening wouldn't confirm the witness account, saying no further information would be released apart from the fact the armed offenders squad was not involved.

Police say a small amount of cash was taken from "a commercial premises on Beach Rd" about 2.30pm today.

Nobody was injured and officers are looking for the robber.

The Mobil is open as normal this evening and there is no police presence.

Anyone with information can contact the North Shore CIB on 09 477 5261.