A Victorian royal commission has been told there is a disparity in support for people with physical health needs and those with mental health needs.

The mental health system is like an abyss where the lack of help and support stands in stark contrast to that for cancer sufferers, a royal commission has been told.

Children as young as five have struggled to care for an unwell parent.

People wanting help have been told they are not 'sick enough' and 'not suicidal enough' to receive care, Victoria's mental health royal commission has been told.

They are some of the tragedies that bring alive what it means to have a broken system, commission chair Penny Armytage said on Tuesday.

On the opening day of the inqury's public hearings, Ms Armytage said the commissioners have been told about the disparity in support for those with physical health needs and those with mental health needs.

"One parent told us about the stark difference in the support they received for one child with cancer and one with a mental health condition," Ms Armytage said.

She said the four commissioners also heard about the desperation of carers, especially young carers, who struggle to 'stand in the gaps' between services and provide support for their loved ones.

"In these stories there is often an acknowledgment that while sometimes our health is a 'lottery', our mental health system should not be."

Counsel assisting the commission Lisa Nichols QC said supply is not keeping up with demand in the state's broken mental health system.

One of the 90 witnesses during the 19-day public hearing will be a woman whose 22-year-old son became seriously ill, spending time in and out of hospital including as an involuntary patient.

"My experience of the mental health system felt like opening a door and seeing a yawning abyss, because of the lack of support and help," the mother told the commission.

Ms Nichols said the commission will also hear from young adults who, as children as young as five and 10, cared for their unwell parents.

"Shopping and cleaning. Cooking and worrying," she said.

Ms Nichols said several witnesses will speak about the so-called "missing middle".

They are the many thousands of Victorians whose needs are too complex for the primary care system alone but who are not sick enough to obtain access to specialist mental health services, Ms Nichols said.

"They are, in the words of one witness, unsung heroes; living with severe symptoms but still often holding jobs and running families."

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