Targeting sickness rates showed it was possible to cut them, said AMs
Sickness rates among NHS staff have dropped, but a group of Welsh assembly members is warning a shake-up of health boards could undo progress.
The audit committee said sickness absence stabilised at 5.3%-5.4% from 2004 to 2008, saving £6m per year.
This compared with 6% absence reported for 2002-03.
Committee chair Jonathan Morgan said the NHS needed to be "mindful" of the possible effect on staff morale and the stress of a planned NHS reorganisation.
The report, a follow-up to its 2004 inquiry, concluded that sickness absence management arrangements in Wales' NHS have improved over recent years, particularly when hospital trusts have focused attention and resources on the issue.
The AMs found that in addition to saving £6m in staff time costs per year, the reduction in NHS staff absence should have helped to avoid some additional spending on replacement staff, such as agency nurses.
However, most trusts could not readily quantify the extent to which replacement staff costs were the result of staff sickness absence, the report said.
The committee found that NHS trusts reported differences in their sickness absence rates, ranging from 4.1% to 7.0%.
The AMs found the "substantial" differences could not be explained as factors outside the NHS, as where trusts had focused particular attention and resources on the issue, it had been possible to reduce sickness absence rates.
Under plans announced in March, the number of local health boards (LHBs) in Wales is being cut from 22 to seven.
Health Minister Edwina Hart said the new LHBs would be established in shadow form from June and fully operational from October.
Audit committee AMs said that although it was still too early to assess the impact of the NHS reorganisation, they were concerned that the process could lead to an increase in sickness absence rates.
They said if the reorganisation was badly managed, it could still have a detrimental impact on morale, while staff turnover and vacancies may also increase the pressure and stress on staff.
Mr Morgan, Conservative AM for Cardiff North, said the NHS in Wales also needed to look at how occupational health services were provided to NHS staff.
He said: "[The NHS] is one of those areas of public service where, because it's so high pressure and there has been lot of change, staff can become demoralised because of these changes.
"It's ironic that for those people who work... within a health context, the NHS has not done sufficiently well through investing in occupational health to help health workers get back into work.
"It means [staff] either go off on the sick very quickly or, while they are on the sick, it becomes more difficult to get back to work.
"The NHS need to be more resourceful as an employer in the way it helps and encourages nursing staff."
Richard Jones, deputy director of RCN Wales, said in addition to physical illnesses, nurses also took sick leave because of exhaustion, stress, bullying and harassment and after violent assaults.
He said: "The RCN wants systems in place to recognise signs and symptoms of illness because early intervention can make a significant difference and can be beneficial.
"Also, if someone does have to go on sick leave, we want to see adequate absence management systems in place to support nurses and others healthcare workers when they return to work."
A spokesman for Ms Hart said she had received a copy of the report and would consider its findings and recommendations.