Technology giant disputes claims that factory staff are working more than 60-hour weeks.
Apple has faced fresh claims of worker rights violations at Chinese factories of one of its suppliers.
Staff at three factories belonging to the Pegatron Group are alleged to be affected by violations of international and Chinese laws and standards.
China Labor Watch (CLW) claims abuses of the laws include underage labour, contract violations and excessive working hours.
The organisation alleges that its investigations revealed "at least 86 labor rights violations, including 36 legal violations and 50 ethical violations".
China has a 49-hour working week limit, but CLW said it found the average working weekly hours in the three factories to be 66 hours, 67 hours and 69 hours in its investigations between March and July.
It added that a factory in Shanghai forced workers to sign forms indicating their overtime hours were less than those actually worked.
CLW found the factory working conditions to be so poor that 30 out of 110 new staff quit within a fortnight of starting at one factory.
Apple and Pegatron said they will investigate the claims immediately.
Apple disputed the average hours findings, saying a June survey found Pegatron employees worked an average 49-hour week.
CLW executive director Li Qiang said, "Our investigations have shown that labour conditions at Pegatron factories are even worse than those at Foxconn factories. Apple has not lived up to its own standards.
"This will lead to Apple’s suppliers abusing labour in order to strengthen their position for receiving orders. In this way, Apple is worsening conditions for workers, not improving them."
Apple came under criticism for similar claims in the past, when one of its biggest suppliers, Foxconn, was accused of violating worker rights.
Foxconn suffered further scrutiny following a spate of staff suicides.
Apple has said audit teams would inspect the three factories this week and has been in close contact with CLW to share findings, adding that it had not been made aware of some of the information in the report.
Apple said it would require Pegatron to reimburse anyone found to have been underpaid or denied overtime payment.