Dec 24 (Reuters) - St Barbara shares plunged nearly 38% on Tuesday and were on track for their worst session in 34 years, after the Papua New Guinea government sent a tax assessment of about A$210 million ($131.15 million) to the Australian gold miner.

The stock fell as much as 37.7% to A$0.19 and was poised for its worst day since October 1990 if current losses held. It also hit its lowest level since June 27.

St Barbara's unit Simberi Gold received the correspondence, which contained an assessment of additional taxes inclusive of a 200% penalty.

The assessment included income tax related to the calculation of asset values since 2006 and its effect on depreciation claims from 2017 to 2021.

St Barbara and Simberi Gold rejected the basis for the assessment and plan to appeal within the statutory 60-day window, with the deadline set for Feb. 17, 2025.

"St Barbara takes the assertion of fraud very seriously and will vigorously defend against these baseless allegations," the company said in a statement, arguing that the claims were an attempt to reopen older tax filings unjustly.

($1 = 1.6013 Australian dollars)

(Reporting by Roshan Thomas in Bengaluru; Editing by Subhranshu Sahu)

((Roshan.Thomas@thomsonreuters.com;))