The parents of baby Tim, born on the side of an Auckland motorway this morning, have described how the city's slow traffic stalled their trip to hospital.

Tim was born in the back seat of his parents' car about 9.45am, on the side of the Southern Motorway near Market Rd.

Mum Olga Ivanova went into labour about 7am and with husband Ivan, headed to hospital.

But the traffic was not on their side. Crawling along at 30 to 35km/h, Mr Ivanova pulled over and phoned for help.

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"I called 111 and they were really nice and walked me through everything," Mr Ivanov told One News today.

"She said, 'the baby's coming out'."

Within 10 minutes, an ambulance arrived.

"I felt relieved because everything was my responsibility. I now had to transfer responsibility to them. We knew that they would take care of her," said Mr Ivanov, who with his wife moved to New Zealand from Russia three years ago.

"It definitely was a crazy experience, not something that you experience every day."

The ambulance officer who helped deliver the baby told the Herald helping to bring a new baby into the world was one of the most rewarding parts of the job.

"It must have been frightening for both the Mum and Dad to be giving birth on the motorway - certainly not part of their birth plan - so to bring them some calm and deliver their baby was really special," St John ambulance officer Dylan Wilde said.

"I'm really pleased that Mum and baby boy are doing so well."

Tim popped out just five minutes after paramedics turned up.

St John ambulance officer Dylan Wilde told One News Tim's umbilical cord was tied around his back, but it was otherwise a routine birth.

"On arrival we found a young female in the back in labour. We did the job and ended up delivering the baby just after our arrival."

Temporary lane closures caused some delays for city-bound traffic but the NZ Transport Agency took it in good humour, tweeting: "It's a boy".

A NZTA spokeswoman said staff at the transport operations centre guided the ambulance to the scene and police blocked off a lane to create a safe work area.

Tim and his mum were taken to Auckland City Hospital but were back home tonight.