The father of three children killed and one seriously injured when the car their mother was driving plunged into a lake in Melbourne's outer-west has said she could not have done it on purpose.

The mother of seven was questioned by police on Thursday after Hanger, a four-year-old girl, and her twin brother Madit were killed along with their one-year-old brother Bol as a result of the crash at Manor Lakes Boulevard at Wyndham Vale on Wednesday afternoon.

Awel, their five-year-old sister, is in hospital in a serious condition, downgraded from critical.

A male family member interviewed outside the hospital was asked by reporters if Awel was awake.

"Yes, she'll be fine," he said.

Community members said the mother Akon Guode, 35, came to Australia from South Sudan to escape war after her first husband died.

Carloads of Sudanese well-wishers also visited Ms Guode's Wyndham Vale home, as did Joseph Manyang, her partner and the father of the four children who were in the Toyota Kluger when it veered off the road.

Mr Manyang told SBS Radio that he did not believe Ms Guode purposely drove the car into the lake.

The South Sudanese community mourn the loss of three children when a car they were in ploughed into a Wyndham Vale lake. ( AAP: Julian Smith )

"My message to the community - because there is a lot of talking around, there is untruths - what I can tell them is they have to wait for the report from the police investigation, that will be the truth," Mr Manyang said.

"What I can say to them, she can't do that, what I believe is she can't do that by herself because she love the kids.

"All the kids, she take care of the kids, she spent her time, always these kids. That's what I can tell them."

Magdalia Acieny, a close relative of Mr Manyang said that Ms Guode "really loved her children".

"They were lovely children, especially the twins, they were like my twins' best friends and now I don't know how to explain it to my twins," Ms Acieny said.

Ms Acieny said she was not aware of any issues with depression in the family.

Father devastated by the children's deaths

Police confirmed on Wednesday that one child died at the scene, another passed away on the way to hospital about 5:30pm (AEST), and the third died in hospital before 7:00pm.

Mr Mayang said he was shocked by loss of his children, who he described as "always happy".

"Especially the twins, because the twins are special to me, but the third one, Bol, is still shy, is one year and a half, always we play together," he said.

"It is a big shock for me to get this information. It has happened to me, to lose three children at once. "

Ms Acieny said she had witnessed his devastation.

Sorry, this video has expired Police confirm crash driver was victims' mother

"He's shattered. Last night he was crying, his nose was bleeding," she said.

Another relative, Amani Alier, said: "He got up and just to see a picture [of his] son on the wall, he started to cry."

Earlier, Superintendent Bateson appealed for anyone with information about the family's situation to come forward.

"We understand that there may be some social, cultural and religious implications impacting on the community, but let me reassure you that we will remain respectful of those issues at all times," he said.

"We do need to understand what was happening in their lives and we do need to understand what their movements were leading up to the incident.

"So we ask, if you have any information that you think may assist us in understanding the situation, that you call Crime Stoppers or pop into your local police station."

He said they were not aware of anyone at this stage who had witnessed the car plunging into the lake.

Ms Guade was taken to hospital immediately after the incident and placed under police guard before she was discharged, before being questioned by police on Thursday.

'There's [nowhere] you'd hit a corner and drive straight ahead'

Police divers retrieve the vehicle that plunged into Lake Gladman in Wyndham Vale. ( AAP: Julian Smith )

Wyndham Vale is about 35 kilometres west of Melbourne's CBD, part of a growth area known for its new suburbs and housing estates.

The road along the lake, an area similar to a recreation reserve with paths for walkers, is very wide and has a median strip between the lanes.

The car appears to have crossed a grassy nature strip, and gone over a footpath and down an embankment before stopping in the middle of the lake.

"The road is probably the main road at the moment that comes into the estate, it's a 50-kilometre-an-hour zone," Mr England said.

"It just sort of sweeps down along the wetlands where the car is, and the major lake on the other side."

"It's a long sweeping bend. So there's [nowhere] you'd hit a corner and drive straight ahead or anything like that."

Mr England said it was a quiet and family-orientated community.

"It's a growing community, full of families and children. The school's not far by, there's 1,800 students there," he said.

"Most people here have kids and it's something you just don't expect to have in your backyard."

Families gather at lakeside

Thomas Kok is a relative of the victims and visited a makeshift memorial in the area on Thursday morning.

"We just came to pay our tributes. We spent all night at the hospital," Mr Kok said.

"It's shocked everyone. I saw the comments coming [from the community]. It makes me feel like people care.

Locals have left toys, flowers and candles beside Lake Gladman, where the car remained submerged overnight.

Mark England, who lives nearby, said it was a "horrific" scene to witness.

"I was at home yesterday and I heard more fire trucks and sirens than I normally hear," Mr England told ABC News Breakfast.

"I thought it might've been a grassfire, so I came down to have a look and I saw the car in the water."

He said he saw police officers and members of the fire brigade trying to resuscitate some of the children.

"After, I reckon it was probably five or 10 minutes later, they must've realised there was another missing person, so they dived into the water and were checking the car and around the reeds ... and they did find someone under the water," he said.

"I think earlier on, when the car first went in, there was reports of people getting into the water and smashing windows to try and get three people out.

"It was pretty horrific."

Locals have created a memorial of flowers and toys near the lake. ( ABC News )

ABC/AAP