A fire raging among 20 000 bales of hay on South Australia's Yorke Peninsula was expected to burn for five days and leave a damages bill of $3.5 million.

The South Australian Country Fire Service was called to the blaze in a paddock at the SP-Hay facility 7 km north-east of Paskeville about 9 pm last Wednesday.

About 200 firefighters attended the blaze, which was believed to have been caused by spontaneous combustion.

Crews back-burned a perimeter around the fire which posed no threat to other property, and have continued to monitor it.

Last July a deliberately-lit fire at the same property destroyed $10 million worth of hay.