In his 2nd Edition of his excellent book, The Numeral Cancellations of New South Wales
, Hugh Freeman lists 18 post offices, including the three mentioned by Bobby De La Rue above, as opened but "cannot at this time be allocated to any numeral".
Numeral 2099 has been recorded for Toolejooa, which opened on 1st July, 1904. Hugh lists Parragundy as the likely candidate for numeral 2100, having opened on the same day as Toolejooa, although no numeral strike has yet been recorded (rated NNR).
Just as many of the previous NNR numerals have subsequently been discovered by diligent philatelists, we may one day find the stamp, piece or cover that reveals the existence and possibly the identity of NSW numeral 2100.
He suggests that Victoria, issuing their last number (2100) over two years later, copied the logic of finishing at that number from New South Wales.