The company's yearly revenue, which was 31% lower than the previous corresponding period at $1.7 billion, was also impacted by a softening in thermal coal prices.
Whitehaven CEO Paul Flynn said the significant contraction in coal prices disproportionately impacted its headline financial results.
"Our immediate focus is on achieving greater efficiency and more consistent operational performance in anticipation of markets rebalancing and price improvements beginning to flow through.
"We are confident about the continuing demand for high quality coal in a more carbon conscious world and the major role it will play as part of the global economic recovery."
Disruption due to drought and bushfires and the impact of unmapped historical underground workings at Whitehaven's Werris Creek also contributed to a 4% drop in FY2020 run-of-mine coal production to 16.6 million tonnes.
Flynn said the NSW government's recent approval of the company's Vickery extension project was a significant achievement, however, the company was bracing for further headwinds brought about by the global COVID-19 pandemic.
The Vickery project is a proposed open cut mine with a 20-year mine life in the Gunnedah Basin with marketable reserves of 178Mt.
The mine will produce a majority metallurgical coal for steel-making, with the balance being high quality thermal coal destined for premium export markets in the Asian region.
"Given continuing short-term economic uncertainty, we remain cautious about expansion and capital allocation," Flynn said.
"I am proud of our team in delivering a record safety result and rounding out the full year with strong operational performance including in the face of challenges brought about by COVID-19."