About 5,500 tonnes of rubbish from a closed landfill were washed out during heavy rain and flooding in March and contaminated Fox River.
DOC's South Westland operations manager Wayne Costello said most of the rubbish was plastic that was entangled with washed-up logs or buried under rocks and silt along Fox River.
"We are very appreciative of the support from the NZDF to help clean up the rubbish alongside DOC staff and our incredible volunteers. People power is what it will take to remove the rubbish from the riverbed," he said.
DOC replaced the Westland District Council as the lead agency co-ordinating the clean-up of the riverbed and coastline in June.
A lot of the rubbish has landed in the Unesco World Heritage area, National Park and marine reserve.
Locals and tourists have been helping in the clean-up and DOC is in the process of finding more volunteers.