Friends of the devoted dad, who runs the Pro Touch Tennis Academy, were keeping a bedside vigil following the tragedy.
Emergency services were called to a home on Maralee Drive after reports of a raging blaze inside the property.
Fire crews pulled Tomkins and her two children from the burning home before bringing the blaze under control. The mother died at the scene.
No one else is believed to have been home at the time of the fire.
Alisha O'Brien, a temporary resident living on the street, said she witnessed the father screaming outside the burning home as the flames took hold.
"They carried out three bodies and they were completely lifeless," she told the Herald Sun.
"When he saw them, he was screaming, 'Oh no!'
"He could not be consoled in any way, he was a complete mess, in total shock and grief."
Another neighbour also reported seeing the father running towards the home.
He teared up as he recalled watching smoke rise from the burning house, telling 9 News that he has watched the two children grow up on the quiet street.
The circumstances leading up to the deadly incident are still being determined; however, police are not looking for anyone else at this time.
One neighbour said residents of the quiet street and tight-knit community were in shock.
"It all unfolded so quickly, it's just a tragic situation," he told the Herald Sun.
"They were a good family."
Jay Rushworth and Luke Dufton, two nearby neighbours, said they were still in disbelief as they laid a toy on the driveway as a tribute.
While the home was gutted by the blaze, the exterior structure of the property showed minimal signs of damage.
O'Brien said fire crews faced heavy flames as they entered the property.
"The flames were as high as a telegraph pole," she said.
"There was a lot of smoke, it looked like the house was going to burn down."
A crowd-funding campaign has been set up by a local man known to the family, raising more than $4000 in a matter of hours for Marshall and the children.
The link for the fundraiser was posted in the Mooroolbark Community Group on Facebook, with one woman thanking the organiser for honouring her best friend.
"Thank you for setting this up for my best friend," she wrote.
"Such a beautiful friend gone too soon."
Comments flooded the post as generous donations poured in for the family.
"Absolute tragedy for all in the family and community, absolutely devastating," one person wrote.