Most have not been briefed on key legislation, including the Children Act and the Anti-Social Behaviour Act. This could leave them "vulnerable to challenge", headteachers believe.
But despite their lack of training, most education welfare officers (EWOs) feel they are well placed to help truants readjust to mainstream schooling.
They say parents are the biggest obstacle to reducing absenteeism rates, and more than half believe truants need vocational or other alternatives to the traditional academic curriculum to improve their learning and attendance.
Last year (2003-4), secondary pupils missed 9.4 per cent of half-day school sessions in Wales - equivalent to nearly 15 days per child - in authorised and unauthorised absence. The figures are down on previous years but the Assembly is unlikely to hit its 2004 target of below 8 per cent.
The Swansea Institute of Higher Education survey, based on questionnaires and interviews with 431 EWOs in Wales and England, is believed to be the first to ask staff with day-to-day responsibility for problem attenders what they think about their work.
Parents who take children on term-time holidays or condone "bunking off" are the biggest obstacles to reducing national and local absenteeism rates, say EWOs.
And while they largely reject fines and jail sentences for parents, they warn that magistrates' failure to deal with those cases that do make it to court are undermining teachers' trust in EWOs.
Anna Brychan, director of the National Association of Head Teachers Cymru, said: "School absence affects pupil achievement badly. Parents must ensure their children benefit as fully as possible from their education."
But she was also concerned about the gaps in EWO training uncovered by the survey, adding: "Training in the latest legal position is essential if EWOs are to operate effectively."
Professor Ken Reid, Swansea Institute's assistant principal, who carried out the survey, said: "Most had not had any training in implementation of the Children Act, the Human Rights Act, the Anti-Social Behaviour Act and the Freedom of Information Act.
"More than a third have received little or no training on the Data Protection Act, the Race Relations Act and the Disability Act. Some are working with children with special needs but haven't done any background work on disabilites."
He believes services are worse in Wales because EWOs have missed out on England-based initiatives which focused on truancy.
An Assembly government spokesperson said it was aware of the issues raised by the survey. It has invited tenders for a comprehensive review of the education welfare service in Wales, which will look at variations in service across the country and the training and accreditation systems.