The cause of a blaze that ripped through bales of plastic waste near Thames earlier this year, remains a mystery.

Fire and Emergency NZ were called to the fire at Smart Environmental Ltd on March 7.

Crews from Thames, Hamilton Ngāruawāhia, Tauranga and Whangamata working to extinguish the blaze, with firefighters working through the night to contain it.

Fire investigators were unable to identify a heat source.

Discarded smoking material, degradation of crushed lithium Ion battery or friction sparks were the most likely causes, according to a fire report, released to Stuff under the Official Information Act.

The cause was ruled as accidental, although "a deliberate fire start was not able to be eliminated", the report said.

-Stuff/ Ruby Nyika