Figures show up to a third of illnesses among NHS Wales staff are caused by stress, depression or anxiety.
At the peak of the coronavirus pandemic, 14,500 staff were absent from work due to one of these mental health conditions.
Figures obtained by BBC Wales suggest that numbers are still not back to pre-pandemic levels.
One union said the figures were “sobering” and that fatigue built up in recent years continued to strain staff.
And in the first nine months of 2023, there were 10,000 stress-related absences across the five health boards and the Welsh Ambulance Service.
Wales' six main health boards, which cover general hospitals and ambulance services, have given details in response to Freedom of Information requests.
These results show that up to 33% of all staff suffer from stress, anxiety, depression and other mental illnesses.
Before the pandemic, around a quarter of absenteeism was due to one of these mental health conditions.
In 2017, there were 9,023 mental health staff absenteeism.
This number rose to more than 13,000 in the primary year of the pandemic and peaked at more than 14,500 in 2021, a 61% increase compared to four years ago.
Although it fell slightly in 2022, it remained at a high level of over 14,200.
The Board of Health reported that the number of cases will continue to decline, albeit slowly, until the end of September 2023.
Aneurin Bevan Health Board reported that 33% of staff absences were related to stress or anxiety in three of the last four years.
Cardiff and Vale Health Board reached 31% in both 2020 and 2021.
Overall, the average illness rate in NHS Wales was 6.4% of all staff in the year to June, down on the previous year.
Welsh Government statisticians say there is a clear downward trend in long-term interest rates.
It peaked during the coronavirus wave and ranged from 7.5% to 7.9% in the most contagious months.
Unison Cymru/Wales regional organizer Tanya Bull said: “Staff absenteeism rates due to stress and other related illnesses are worrying but perhaps not surprising.”
“NHS workers have felt the brunt of the pandemic, often putting themselves and their families at risk to provide vital care to people in Wales.
“Fatigue built up over years of the pandemic continues to strain staff as they face new challenges.
“Coupled with the cost of living crisis, it is no wonder NHS staff continue to experience burnout and stress.
“Challenges with staffing and financial pressures continue in health boards, with pay for NHS Wales staff not keeping pace with increases in the cost of living, creating further uncertainty for household finances.”
The Welsh Government said it recognized staff as the backbone of health and care services and thanked them for their continued efforts.
It said it would continue to work with NHS health boards, trusts and trade unions to understand how targeted funding and services could be provided to complement the support provided by employers.