The Council of Trade Unions says lower benefit figures do not show where people are going and that more data is needed.
Statistics from this year's March quarter show over 280,000 people received a benefit, down almost 4 percent since this time last year, and the lowest since 2009.
Over 40 percent of those on a benefit are receiving Jobseeker Support, and a quarter receive Sole Parent Support.
But senior policy analyst at the Council of Trade Unions, Eileen Brown, said there needed to be more data on those leaving the benefit.
"I think there needs to be much better evaluation and much better tracking - that would also include what kind of jobs people are going into, how long they're staying off the benefit and support for them to ensure they don't return to the benefit."
Ms Brown said the lower figures did not necessarily mean more people are finding work.
"We have to be very careful about assessing this statement.
"One, think about what that means, are people actually coming off benefit. Where are they going? And thirdly, what kind of work are they going in to? Are people actually going into jobs, or are they coming off benefit? They're two different things."
She said CTU had serious questions about why benefit figures are going down, and more information should be available.