The ASX 100 contains the top 100 companies by way of float-adjusted market capitalisation and accounts for three quarters of the Australian equity market.

NIB was added as part of the Australian Stock Exchange's September 2019 quarterly rebalance of the S&P/ASX Indices.

nib managing director Mark Fitzgibbon said the change reflected the insurer's strong ongoing performance.

"It's been a fantastic journey since our listing on the ASX in 2007 to now being included as one of top 100 companies in the country," Mr Fitzgibbon said.

"We continue to deliver upon our commitment to meeting the needs of our members and travellers wherever they are in the world, which translates into commercial results.

"We've also delivered very good returns for shareholders, with a total shareholder return since listing of 1,701% compared to 66% for the ASX200. In addition, our share price has gone from 0.85c at listing to more than $7.00 currently," he said.

The change is effective at the open of trading on 23 September 2019.