Visitors from Echuca had to call triple zero when their baby was accidentally locked in their car in Longfield Street on Sunday.

Stawell Ambulance Branch Team Manager Wayne Rice responded to the call. He said he was able to free the baby quickly by breaking a small window to unlock the door.

The baby was monitored in the air-conditioned ambulance and assessed to be well for the onward journey.

"The baby was fine. We checked his pulse, respiration rate and colour. He was alert and orientated," Mr Rice said

It is estimated the baby had been locked in the car for about eight minutes. Had the incident occurred outside town the outcome could have been devastating.

"In the countryside you wouldn't get an ambulance and police out there as quick as that. Your mobile phone may be locked inside the car too," Mr Rice said.

Ambulance Victoria testing has found cars reach an internal temperature of 44 degrees within ten minutes and 60 degrees within 20 minutes on a 29 degree day.

Mr Rice said the incident should serve as a reminder not to leave children or pets unattended in a vehicle, even for quick stops to get petrol or milk.

With this week's heatwave the worst since 2009, Ambulance Victoria activated its emergency response plan on Tuesday to cope with the expected increase in triple zero (000) calls.

"Hot weather including high overnight temperatures in 2009 led to an extra 374 deaths across Victoria in a week," said Ambulance Victoria's Manager of Emergency Management Paul Holman.

"We urge the community to avoid exercising or spending long periods outside in the heat of the day and to check on elderly and frail relatives and neighbours to make sure they are okay," Mr Holman said.

Ambulance Victoria tips for avoiding heatstroke:

Stay indoors or in the shade.

Avoid exertion in the heat of the day.

Drink plenty of water.

Wear light, protective clothing.

Never leave people or pets in cars.

Use air-conditioning/cooling if you have it.