The factory cited in the China Labor Watch report is owned by manufacturing giant Pegatron and employs around 100,000 people. Apple's
lists the factory as building both smartphones and tablets.
In September, the facility was at the start of its busiest period of the year, manufacturing Apple's latest smartphones, the iPhone 6S and iPhone 6S Plus. During that time, China Labor Watch sent an investigator to work there undercover.
The investigator claimed that workers earn about $1.85 (AU$2.55 or £1.20) per hour and pull significant overtime hours to make enough money to cover living expenses. The report further claimed that the standard shift was nine hours a day, but that starting in September staff worked an additional minimum of 20 hours of overtime each week, usually split between an extra two hours each weekday and one 10-hour shift on Saturdays. With overtime accounted for, the factory's workers earn about $753 (AU$1,045 or £490) in monthly wages.
This overtime was essentially the minimum, according to the investigator, who claimed to be told by a trainer that working eight-hour shifts five days a week "does not conform to our hiring requirements."
Apple declined to comment on China Labor Watch's report. Pegatron was not available for comment.
In recent years, Apple has sought to cap the excess work hours at the factories, with its guidelines for manufacturers stating a 60-hour maximum workweek. Last year, 92 percent of the workers Apple monitored in its supply chain were compliant, according to the company, but in September 2014, when Apple was busy readying the iPhone 6 line for sale, the percentage fell to just over 75 percent.
China Labor Watch found that this year the compliance rate was lower at the Shanghai Pegatron factory. The group collected pay stubs from 76 workers in September and found that only 42 percent of the employees had worked 60 hours a week or less.
It's not the first time China Labor Watch has spoken out against working conditions in iPhone manufacturer factories. In 2013, the group slammed Apple over low pay for the workers who make its devices.
This time around, the group said that conditions had improved in a few areas, most notably the hiring process. It said that in 2013 Pegatron was "explicitly" discriminatory against people over 35, people with dyed hair and those of Tibetan or Uyghur ethnicity, but none of that was present in 2015. There were also "partial" improvements in regard to sick leave and resignation, among other areas, according to the report.