Australian uranium miner Boss Energy's shares plunged 42% in Sydney trading after the uranium developer issued a production guidance for 2026, warning of rising costs and flagging uncertainty over achieving nameplate production capacity at its Honeymoon operation in South Australia.

The ASX-listed company on Monday released its fiscal year 2026 guidance, setting production at 1.6-million pounds of uranium oxide (U₃O₈) and forecasting C1 cash costs of A$41/lb to A$45/lb ($27/lb to $29/lb), with all-in sustaining costs (AISC) of A$64/lb to A$70/lb ($41/lb to $45/lb).

Honeymoon's production totalled 872 607 lb in 2025, with second-half C1 cash costs of A$35/lb. The miner realised an average price of A$109/lb ($71/lb).

The company said Honeymoon was expected to remain cashflow positive, with production supported by nine wellfields in operation by June 2026.

While Boss said it exceeded production and cost guidance in the 2025 financial year, the company disclosed that recent delineation drilling at East Kalkaroo, where future wellfields will be located, has shown less continuity of mineralised horizons than previously assumed in its 2020 feasibility study and 2021 enhanced feasibility study (EFS).

“This potentially means that additional injection (and extraction) wells may have to be installed, leading to an increase in sustaining capital expenditure per pound when compared to the studies,” the company said. Total sustaining and infrastructure capital expenditure for 2026 is estimated at A$56-million to A$62-million.

"Now that Boss has been able to analyse the initial 12 months of actual performance and design for wellfields B1 to B5, and assess recent delineation drill results for wellfield development at East Kalkaroo (B6 to B9), Boss has identified potential challenges that may arise in achieving nameplate capacity as previously outlined in the EFS. This is largely due to the potential for less continuity of mineralisation and leachability."

The EFS targets nameplate capacity of 2.45-million pounds a year.

The company further stated that an independent review would be conducted “to determine the extent to which the above affects EFS assumptions,” and said it would keep the market informed.

At Alta Mesa, in Texas, where Boss holds a 30% interest, 203 798 lb were produced on a 100% basis. 

Shares in Boss traded at A$1.96 apiece on Monday.