The Parliamentary Library on behalf of Mr Clark got the statistics for 0-11 years based on the Ministry of Health's figures, which "shocked" Mr Clark.
56.3 per cent were seen within three weeks and 86.5 per cent were seen within eight weeks.
"This means 44 per cent of 0-11 years were not seen within the three-week period. This means every second child is missing out."
Hawke's Bay was on par with the national average.
55.7 per cent were seen within three weeks and 87 per cent were seen within eight weeks.
Mr Clark said nationally 2658 children aged under 12 had to wait more than three weeks to be seen by mental health services.
"A 7-year old child with anxiety, depression or other issues who is possibly suicidal should not be waiting three weeks, let alone eight weeks to be seen by a medical professional."
However Hawke's Bay's District Health Board medical director of mental health Dr Simon Shaw said this was an "incorrect" and "misleading" statement by Mr Clark, as no one who needs urgent assistance waits.
"Children can be seen urgently during the day, night and in weekends. There is a group of clinicians specifically available for urgent assessment and urgent follow-up care every day."
Dr Shaw said a huge amount of work on improving mental health services for both children and adults had been done in Hawke's Bay.
"Children are triaged and assessed by one of HBDHB's Child Adolescent and Family Service Team, and then appropriately seen, based on clinical assessment and need."
The Ministry of Health's director of mental health, Dr John Crawshaw, said ensuring access to mental health provision was a priority for the ministry, particularly around youth.
"The Ministry's aim, along with the other agencies working to support mental health, is to ensure that New Zealanders receive quality care.
"Although we're seeing an increased number of people accessing specialist mental health and addiction services, it's an increase consistent with international trends."
Since 2008-09 mental health expenditure by the country's 20 DHBs has increased 18 per cent and Budget 2017 included an extra $224 million over four years in mental health services including $124m in innovative approaches.
Mr Clark said a review of Mental Health was needed, including the implementation of Labour's plan for primary mental health teams to make sure children got the care they deserved.
"We have already established a two-year pilot programme of primary mental health teams at eight sites across the country to work with GPs, PHOs, DHBs and mental health NGOs.
"Each mental health team will comprise mental health service co-ordinators, who will be doctors or other medical professionals. It will be their job to help people navigate the system, which mean patients will deal with the same health professionals throughout."
