Two other activists involved in the investigation are missing, CLW said.
According to the group, the supplier also makes products for other Western brands including Karl Lagerfeld, Coach and Nine West.
CLW called on US President Trump to press for the release of Mr Hua.
There has been no comment so far from Chinese authorities, the Trump administration or the Ivanka Trump brand.
'I was shocked'
CLW said Mr Hua was detained in Jiangxi province on suspicion of illegally using eavesdropping equipment but that this had "no factual basis".
The group's executive director, Li Qiang, told the BBC the firm Mr Hua had been investigating, Huajian Group, had "violated workers' rights in ways that included: forced overtime, wages lower than China's legal minimum wage, managers' verbal abuse of workers, and violations of women's rights".
In 17 years of investigating Chinese factories it was "the first time any our investigators have been detained under a criminal process," Mr Li said.
Mr Hua's wife told the BBC she had lost contact with him on 28 May, but then police contacted her on Tuesday to say he was being held.
"I was shocked and freaked out," Deng Guimian said.
"I only know that my husband was investigating a factory based in Ganzhou, Jiangxi province. That factory manufactures US president's daughter's product," she said.
"I think he was detained for that. Police told me that my husband has been detained for illegal monitoring activities."
The other two men - named as Li Zhao and Su Heng - were working undercover at Huajian factories in Dongguan in Guangdong province and in Ganzhou.
China has seen frequent crackdowns on labour rights activists and rights lawyers in recent years, with campaigners being harassed, arrested or jailed.
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