Professor Richard Daugherty led the assessment review group, whose 2004 proposals led to the abolition of national tests for 11 and 14-year-olds in Wales.
Teachers' assessments are now the only national measure of how well children are doing at the end of each key stage. Pilot work has begun on ensuring their judgements are consistent within and between schools, by improving moderation arrangements and accrediting systems in secondary schools.
But some unions have warned the changes will mean more work and meetings away from school.
Professor Daugherty, speaking at a conference in Cardiff this week, suggested multi-tasking teachers were already overburdened with daily demands.
"Are we continuing to make a set of demands on them in relation to assessment, and the range of skills required, that is unrealistic?" he asked.
The key is to make the "right demands at the right time on teachers who have limited time for assessment", he said.
Policy-makers need to be clear about what assessments are for, so teachers can identify with them and see their educational value.
And managers at every level need to show that the information generated by teachers is useful to the system and for learners.
"The debate isn't about assessment as such, it's about what assessment is contributing to the experience of the learner, so that they will be a better learner," said Professor Daugherty.
Heledd Hayes, education officer with the National Union of Teachers Cymru, said there were concerns about workload. But she said getting rid of testing in Wales should free up resources to support moderation work.
"If the huge spending on tests was redistributed so that teachers feel they have the time and support for moderation, and that they are learning something new, it should be fine," she said.
Professor Daugherty was speaking at a conference on the latest proposals from the influential Assessment Reform Group.
The group, best known for promoting formative assessment ("assessment FOR learning") by teachers, has published new research saying teachers are also best placed to deliver assessment OF learning - such as for measuring the progress of all pupils in schools, LEAs or nationally.
Its latest research says teacher assessments, properly moderated, can provide a better and broader measure of how well children are doing than one-off tests.
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