MinRes said the sale price for 6% spodumene concentrate shipments for the current quarter would be US$608.95 per dry metric tonne compared with US$682.38/dmt for the June quarter.
MinRes said Mt Marion, a 50:50 joint venture with Ganfeng, remained a "profitable and highly valuable long-life lithium asset".
The company said last week Mt Marion shipped 81,000 wet metric tonnes of concentrate, 69% of which was 6% and the remainder 4%, during the June quarter.
MinRes did not report cash operating costs.
The price for 6% spodumene concentrate has been gradually falling.
MinRes was receiving $1070.85/dmt in the September quarter of last year, $930.80/dmt in the December 2018 quarter and $791.84/dmt in the March 2019 quarter.
The lower September quarter figure is in line with Pilbara Minerals' commentary last week of a current price of $580-640/t. Pilbara's average price during the June quarter was $644/t.
MinRes shares dropped 1.4% to A$16.41 in early trade.