The union warns that new arrangements could add 15 hours to the working week of teachers as they spend less quality time with their teachers and more on assessing and moderating their work.
It comes as talks between the teaching union and the Assembly government to quell fears of extra workload and "nightmare" bureaucracy under the new arrangements break down.
Tim Cox, the union's Vale of Glamorgan secretary, repeated earlier threats by members to boycott the new assessment arrangements, following last week's meeting with officials.
Moves toward internal assessment at KS2 and external accreditation at KS3 were recommended last year in the Assembly government document Future Assessment Arrangements for Key Stages 2 and 3, and later approved. Testing for KS2 and 3 pupils has been gradually phased out in Wales since 2005. It was optional last year but abolished since January this year.
Ninety-four per cent of NASUWT Cymru members recently polled say they would prefer a simple and more streamlined assessment system than planned.
Meanwhile, growing concern over a lack of funding for the play-led foundation phase could also lead to a teacher boycott.
Roll-out throughout Wales of the initiative for under-fives happens this September. The National Association of Head Teachers Cymru will meet in Cardiff today to discuss funding.
All of Wales's directors of education should know soon how much cash they will be able to pass on to schools this week.
Neil Hardee, Cardiff Council schools support officer, estimated last week that the city will get around pound;2.5 million from the government, a shortfall of around pound;1.5m on its estimates.