They estimated the total area of the blaze at some 4,000 square metres, causing a large part of the roof to collapse and even spreading to nearby trees.
It is the latest indicator to point to robust economic growth in the January-March quarter, after recent data showed Chinese manufacturing activity was strong last month.
Wistron failed to implement proper working hour management processes, which "led to payment delays for some workers in October and November," Apple said.
The Competition Authority website said Hyundai had made a request to take full control of the factory, which produces Hyundai models such as the i10 and i20.