The Hunter-founded, national insurer lodged a copy of a letter to the ASX and shareholders from its chairman, Steve Crane, in relation to Covid-19 on Monday.

"The Covid-19 threat has created profound uncertainty about NIB's previously communicated commercial outlook and financial year 2020 market guidance. So much so that it has become necessary to withdraw that market guidance," it said, adding updates would follow "if circumstances warrant."

"The only certainty is that Covid-19 will, at least for the near term, have a significant impact upon the private health industry and our business," it said.

NIB's response on Monday came one month after it posted its half-year 2020 results, including an underlying operating operating profit of $83.2 million - a drop of 27.2 per cent compared to a year ago.

READ MORE: "We didn't anticipate getting hit so hard": Fitzgibbon

At the time, NIB managing director Mark Fitzgibbon described the results for the first six months to December, 2019, as "disappointing" but noted the challenging market conditions.

In its letter to shareholders on Monday, Mr Crane said NIB had made its best endeavours to understand possible developments and drawn upon quantitive, qualitative, anecdotal and historical data to form its assumptions in the statement.

"However, our insights should be regarded as directional and speculative," he said.

He said "easily the most significant near term impact" would be the likely consequence that Covid-19 would discourage discretionary treatment within private hospitals.

More information can be seen at NIB's website.