Dad Josh Deen said the couple were on their way to the Royal Women's Hospital when he pulled over 300 metres from the Millers Road exit about 9.30pm on Saturday after his partner, Courtesy Hove, said she needed to "push now".

"I just made it off the M80 and had to pull over into the emergency lane when she said: 'I'm going to have the baby'," he said.

"So I had to call triple zero and tried to get the ambulance to come, but I couldn't wait for that, so the person over the phone had to talk me through it, how to use my hands and how to catch the baby."

Mr Deen said that despite being nervous, he was able to focus and deliver the baby safely.

"I remained calm, I tried not to make the mother panic ... and then ambulance arrived 15 minutes after the birth."

Ms Hove, who was energetically roaming her maternity room 13 hours after the birth, said she felt great.

"It was very quick - one push," she told Fairfax Media.

"In the future, we'll drive past the freeway and say [to our daughter]: 'Look, that's where you were born'," Ms Hove said.

The couple, who already have a 14-month daughter called Egypt, said it was a much quicker birth than the first. They named the baby Sierra - after the country Sierra Leone - in honour of Ms Hove's African descent.

"I'm from Africa, so I try to name the kids after every country I want to go to and look at in Africa," she said.

"I came here when I was 13, so I haven't travelled around, but this is en encouragement to take the kids there."

Ambulance Victoria spokesman Ray Rowe said paramedics arrived to find the baby safe and well with her parents.

"The dad had already started to assist his wife when paramedics arrived."