Phoebe Alexander, 21, was driving along the Calder Highway in Big Hill, when she noticed her engine was making strange noises about 3pm.

"I pulled over and was having a sad moment when two motorcyclists came running and were like, 'Get out - it's on fire!'" she said.

Moose Boyd and Linton Nook White were the two men who came to Ms Alexander's assistance.

Mr Boyd said the pair were riding toward Bendigo when they saw the bottom of Ms Alexander's car was on fire.

"We screamed at her to get out of the car because she didn't know what was going on, she didn't realise it was on fire," Mr Boyd said.

"She probably thought 'what are these hairy bikies doing here?'"

Mr Boyd said the engine fan was on fire and a bit of plastic, which had dislodged from the engine and was dangling underneath the car, was also on fire.

He said he tried to pull out the plastic, to no avail.

A few seconds later,12-year-old Alana Gray bolted down from nearby Big Hill Vineyard with a fire extinguisher.

The men on motorcycles then used the extinguisher to put the fire out.

Alana said the bottom part of the car was in flames when she arrived.

"It was a bit scary seeing the car in flames," she said.

"It was pretty scary seeing it but once it was all done the firies came to check over the car to make sure it wouldn't explode or anything like that."

Lockwood CFA Lieutenant David Hutchings, who attended the scene, said the engine was intact and the car - a 2005 Volkswagon Polo Club - had minimal damage from the incident.

He said Ms Alexander was lucky the Good Samaritans came to her rescue.

"Even if it takes the CFA 90 seconds or two minutes to turn out (to a car fire), the engine could be a black shell (by the time the CFA arrives)," he said.

He said all drivers should keep a fire extinguisher in their car boot.

Ms Alexander said she was grateful to Mr Boyd and Mr White and to Alana.

"I'm really, really lucky," she said.

"Those two motorcyclists were amazingly helpful ... They were top blokes."

Mr Boyd said about 40 cars passed the scene without stopping, but he was always happy to help someone in need.

"You see someone who needs help and you just do it," he said.

Mr Hutchings said he was unsure what caused the fire.