Work and Income is defending its use of consent forms to question doctors about people claiming an invalid's benefit.
A barrister in privacy law says the process misleads patients about the consent they have given to disclose information, and places doctors in an unfair position.
Ministry of Social Development figures for the December quarter show only 23 new people are on the invalid's benefit compared with at least 400 in previous periods.
Work and Income acting deputy chief executive Mike Smith says the department is not getting tougher with claimants and the consent form process was worked through with the Privacy Commission and is correct.
The ministry says no conclusions can be drawn from figures that show a smaller increase in people on the invalid's benefit in December quarter.
Beneficiary advocates have claimed that people on the invalid's benefit are being pushed onto the sickness benefit.
But the ministry's deputy head of policy, Sue Mackwell, says invalid's benefits are affected by demographics and the numbers can fluctuate between quarters.
Ms Mackwell says the recent figures may just be a blip.
