Arson chemists from Melbourne are investigating the cause of a Clear Lake house fire from Sunday night.

Horsham Criminal Investigation Unit Senior Constable Nathaniel Warren said the chemists were called in to determine the exact cause of the fire.

"We are unsure of where it started and how so they can clear that up for us," he said.

Earlier

A Clear Lake family is reeling after a fire engulfed and destroyed their home on Sunday evening.

CFA crews were called to a structure fire at 61 Scott Hair Watsons Road, Clear Lake at about 5pm on Sunday, with the house "fully engulfed" when they arrived.

"Six brigades and about 40 firefighters attended, finding the whole house on fire when they got there," CFA District 17 operations manager Craig Brittain said.

"The five occupants of the house evacuated safely, with Ambulance Victoria called to assist but not required.

"Crews were unable to save the house due to its age and the ferocity of the fire."

Mr Brittain said investigations into the cause of the fire are still continuing.

"We will keep looking into the cause and origin of the fire but at this stage it does not appear suspicious," he said.

Tim Shaw, wife Kirstie and children Eli, 15, Alex, 12 and two-month-old Evie were watching a movie on Sunday afternoon when they noticed smoke billowing from the house. 

"We were having a quiet Sunday afternoon when I got up to go to the kitchen and saw a thick, white smoke out the front window," Mr Shaw said.

"I ran outside and started to fight the fire coming from under the house with a garden hose. I was able to keep the flames off the door and got my family free, but by that time the fire had already encircled the entire house.

"Within about five minutes the hose was completely engulfed. I was able to get my family out but we lost the family dog, cat and kitten."

Mr Shaw said he and his family "lost everything" in the fire.

"Family photos, everything we own - it's all gone. All I have is a long sleeve t-shirt and a pair of shorts to my name," he said.

"In 2016 there was a horrific accident in the same house, and we lost our three-year-old son Blake. Now his ashes and every possession are completely gone.

"We don't know which way to look or where to get support. We have spoken to council about an emergency funding grant but the whole situation is just so overwhelming."

The family has a car but its keys were lost in the fire. 

Mr Shaw said he believes a build up of fairy grass in the area and underneath his house contributed to the fire. 

More to come.