Interviews are due to start soon with a sample of 3,000 clients, including the parents with care of the children - usually the mother - and the absent parents.
But two aggrieved fathers, who face a four-fold increase in payments, who heard about the survey and asked to be included were told by Mori they could not take part because the sample had been selected.
Mike Pimblott from Southampton, who wanted to be included in the survey, is co-ordinating a network of groups to campaign against the new assessments being imposed since the agency was set up in April.
Over the next three years the agency will be assessing payments made to parents with care of the children who are on income support or family credit or who have no court order. From 1996 mothers who are not on income support and who already have a court settlement can apply to the agency to have the payment increased. More than 430,000 absent parents who were already paying maintenance could face increased bills.
Ministers have consistently emphasised the agency was to track down 'feckless' absent fathers who did not pay anything towards the upkeep of their children. But many who do not pay maintenance are unemployed and on income support and so are exempt from paying.
Last month a leaked memo showed staff were being encouraged to target middle-class parents who were already paying maintenance to help meet the Government's target of saving pounds 530m this year.
Since assessments speeded up last month, more than 1,000 aggrieved fathers have joined action groups.
Many fathers who have previousy been paying agreed sums, taking into account their financial circumstances and arrangements such as giving their ex-wife the former marital home, or most of the equity from it, now find this is not being taken into account by the agency. Many are being ordered to pay up to one-third of their salary.
In some cases, if the increased payments take the ex-wife above income support level they are left worse off because the Treasury claws back not just income support, but also other benefits such as some housing benefit and council tax benefit and free prescriptions, dental care and school meals.
The National Association of Citizens' Advice Bureaux has been contacted by hundreds of people seeking advice, including ex-wives and new partners, whose income is taken into account when the agency decides whether the father can afford the assessment.
The association says the assessment formula is flawed because it fails to take account of many of the absent parent's outgoings such as debts, the cost of travel to work, bank loans or hire purchase.
One man from Leicester told the Independent that after he and his wife split up in 1991 he took out a loan to pay for a pounds 12,000 deposit on a flat in London for his ex-wife. To pay for half the pounds 600-a-month child care costs for his four-year-old son he took out another loan - which will cost pounds 5,000 over four years to repay.
He was paying pounds 100 a month maintenance, but the agency has increased this to pounds 375 a month. He said the only way he could afford to pay was to sell his house, which was worth pounds 80,000. After the pounds 30,000 mortgage, he would be left with pounds 50,000 equity of which his ex-wife wanted pounds 27,000.
The man, who is on sickness leave from his work because of the worry, said: 'I went into a state of shock . . . I feel criminalised because I can't afford to pay the new amount. She earns more money than me but this new agency gives her carte blanche to take me to the cleaners.'