It was suggested that the cause maybe the banana rust thrips, Chaetanaphothrips signipennis. Is that insect known to be in the country? The write said, in a later post, that the trips has not been recorded in Mauritius ,and that the banana variety is Dwarf Cavendish. Another member agreed that it could be thrips, but suggested that the fruit are examined on the bunch where they touch to see what if any insects are present. If thrips are present they will be small, about 1 mm or less, and the immature stages will be white or yellow, and adults a little darker where the wings sit in the centre of the abdomen. The thrips move rapidly to hide when fruit are separated.

There are a number of genera and species which produce this damage on banana fruit , so that if you find thrips present a specialist thrips taxonomist will need to determine which species is present. The various species all seem to produce quite similar damage to my knowledge. A useful reference for you is:

http://www.daff.qld.gov.au/plants/fruit-and-vegetables/a-z-list-of-horticultural-insect-pests/banana-rust-thrips

There is a physiological disorder called maturity bronzing which could be confused with rust thrips damage hence the need to closely check for these insects.