On Monday afternoon, relatives from both sides of the family gathered at the Royal Children's Hospital as they cared for the Kilmister children and Paul.

"We’ve been totally devastated by the events of the past few days, and it’s going to be quite some time before we fully appreciate the loss of Amanda and Harrison in such horrendous circumstances," Ms Kilmister's brother Daniel Gwynne said.

"Right now our families are banding together to rally behind Paul and Austin, as we slowly understand the full extent of their conditions and in anticipation of a long, difficult recovery."

The family said Morgan had been discharged from hospital and baby Austin was in a serious but stable condition with a neck fracture and possible spinal injuries.

The boys' father remains unconscious.

"We are preparing ourselves to support [Paul] as he learns of the tragic events of this weekend and the incredible loss he’s going to feel as a husband and a father," Mr Gwynne said.

Mr Kilmister's mother, Christine, said they planned to be by his side when he woke.

"When he comes out, we will be with him and if we need social workers with us to broach it... because he's going to know as soon as we start talking to him," she said.

"Young Felix has been talking to his Dad, even though he's unconscious, about Austin, Morgan, but he hasn't mentioned Harrison or Mummy... he's not going to cope, he's so devoted and his children and wife were his life, that's what he worked for."

Police said on Saturday that the truck driver involved in the crash was on the wrong side of the road. It was reportedly the driver's first shift for the company.

The investigation will look at whether distraction, fatigue, speed or drugs played a part. The 31-year-old Mooroopna man has been interviewed by police and was released pending further inquiries.

Mr Gwynne's said on Monday he hoped the truck driver was being supported by family.

"I'd like you to know that our thoughts are with you," he said.

"Regardless of blame or circumstance, an incident that happened in a few short minutes is likely to affect you for quite some time to come. I hope that you have the love and support of your family to help you through this time."

Family members described Ms Kilmister's complete devotion to her four sons, saying she was as an "incredibly warm and nurturing" mother who invested every part of her life to caring for her children.

"Amanda was a fairly athletic person when she younger, into sport, very popular, very nice personality in school. And Amanda and Paul, they were into water skiing and did plenty of activity with the boys," Ms Kilmister's father Noel Gwynne said.

Harrison, 12, was described as a gentle and studious boy.

"One of Harrison's prep teachers said she was only talking to another teacher on Friday explaining about this amazing child," Christine, Harrison's grandmother said.

"We were in prep singing, 'There's a hole in my bucket', and Harrison said, 'Excuse me, why don't you just buy a new bucket? They are only 69 cents.' That's Harrison. Harrison was an A+ student, he was just meant for great things in this world, he's such a gentle person."

A Go Fund Me is being set up to raise funds for the family's recovery.

The head-on collision was one of three fatal crashes that killed four people at the Melbourne Cup weekend.