Hospital data reveals an increased use of children's mental health services after Sydney's first lockdown in 2020.

In a joint study between UNSW Sydney and the Sydney Children's Hospitals Network (SCHN), researchers compared the observed and predicted numbers of children's admission to inpatient services and hospital emergency departments (ED) between January 2020 and February 2021.

They found that once lockdown was implemented in March 2020, there was a noticeable drop overall in hospital admission and ED attendance for chronic conditions, acute infections and injury.

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Mental illness and cancer were the only two two conditions that did not see a reduction in hospital presentations after lockdown started in March 2020.

When lockdown ended in late May 2020 and restrictions began to be eased, the use of children's mental health related rose sharply beyond pre-pandemic levels.

"When we compared the actual number of children and adolescents using hospital mental health services with the predicted number based on the pre-pandemic data, they were up by 30 to 55 per cent from June 2020 to February 2021," study lead author Dr Nan Hu says.

The demand has put pressure on service providers, who are unable to keep up.

Those seeking mental health support in the Illawarra region of southern NSW for the first time face wait times of up to six weeks, if they are able to find a provider at all.

One local mum said she had been in touch with half a dozen mental health services to provide NDIS-covered mental health support for her child. None had vacancies available; one service was able to offer her a spot on their wait-list.

"You have to treat advocating for your children like a full-time job, and many parents just don't have the resources to do that," she said.

Michelle Montgomery, of New Leaf Counselling and Wellbeing, said there was a gap in Medicare funding which made it harder for parents to access the support they need for their children.

"As a registered counsellor with the Australian Counselling Association I am funded to provide support for children with an NDIS plan, but not covered by Medicare to process mental health care plans," she said.

"The majority of my clients are under the NDIS, but through COVID everyone who got in touch needing help were teenagers. There is such a backlog, and without Medicare, many parents can't afford it. Even as a registered psychologist you have to have certain qualifications to be covered by Medicare, and people are crying out for help."

She encouraged parents to take advantage of online resources available from organisations like BeyondBlue and Headspace while in the process of finding in-person support for their child.

If you're struggling call Lifeline on 13 11 14; Kids Helpline on 1800 551 800 or Suicide Call Back Service on 1300 659 467.