Shortages resulting from absent staff disrupted learning and caused some students to drop out of courses, the Wales Audit Office said.
The study also uncovered widespread dissatisfaction with occupational health arrangements, with some staff waiting three months for an appointment.
Sickness absence ranged from 4.8 to 16 days a year among Wales' 25 colleges. It averaged 8.9 days a year, above the private sector rate of 7.8 days, but below the Welsh Assembly at 10.6 days. Jeremy Colman, auditor general for Wales, said: "My report shows there is scope for institutions to tighten up their management of sickness absence.
"Aside from savings to the public purse, implementations of my recommendations will also lead to more appropriate and sensitive treatment of staff, less disruption for students and help to improve the quality of learning in further education."
Mr Colman demanded colleges introduce better sickness absence policies and procedures, monitoring and target setting. The report found more than a fifth of sickness absences were not properly certified. Although nine institutions said they carried out return-to-work interviews, only 16 per cent of staff said they had had them.
But FE absenteeism compared favourably with the UK-wide civil service rate of 10 days, Welsh local government at 12 days and the Welsh NHS at 15.6 days.
Phil Markham, regional support official for Natfhe, the lecturers' union, said: "While these statistics suggest levels of sickness absence in colleges in Wales are lower than for most public services, Natfhe would welcome actions which would identify and reduce the causes of sickness absence.
"I would like to see sickness absence monitored in a transparent way which helps to identify its causes."
The WAO also found that a third of one-day absences were on a Monday and called on ELWa, the Welsh FE funding body, to identify and promote the best ways of tackling the problem.
"We will continue to work with further education institutions to improve their management of sickness absence through implementation of the recommendations outlined in the report," said an ELWa spokeswoman.
John Graystone, chief executive of fforwm, the Welsh colleges' association, said: "fforwm welcomes the fact that staff sickness absence at FE colleges is better than other areas of the public sector, but we recognise there is much room for improvement. It is vital that learners have consistent access to quality provision."