Just over a quarter of the National Disability Insurance Agency's workforce is labour hire, the Community and Public Sector Union has told a Senate committee.

Many of those contractors earned $10 or so less an hour for the same job compared to their public service counterparts, union deputy secretary Beth Vincent-Pietsch said on Wednesday.

"They are frequently told by their (labour hire) agency that they are prohibited from discussing their pay rates and it's quite intimidating for them to speak up in any way abut their pay," Ms Vincent-Pietsch said.

"To meet the needs of the community that these agencies were built to deliver to, the labour hire needs to end.

"These agencies need to be properly funded and resourced and with a proportionate increase to permanent staffing."

She also said "unworkable" performance targets and insecure work created a high staff turnover, which sent money and time spent training people down the drain.

NDIA official Lisa Studdert told the committee the agency used 1555 labour hire workers as of June 30.

The agency has public service staffing caps imposed on it.

"I would probably be an unusual senior executive if I didn't say the APS cap felt constraining some days, but we have a very capable labour hire workforce too," she said.

The agency's chief people officer, Hamish Aikman, said it was the first he'd heard of complaints labour hire workers were ordered not to discuss their pay.

No such direction came from the NDIA, he said.

Australian Associated Press