The Bank Bill  Swap Rate, commonly known as BBSW,  is simply the short  term swap rate. In Australia, BBSW is the term used for interest  rate swaps of six months or less, anything dated longer than six months is  simply referred to as a swap rate. 
While BBSW has  many uses, for fixed income investors its main relevance is as a benchmark upon  which we can evaluate floating rate bonds or investments.
Swap
  An interest rate swap is a financial instrument where one  entity swaps a stream of floating interest payments for another entity’s fixed  interest payments. In practice, they do not make two payments. Rather, a single  net payment is made.
BBSW 
As of 1 January 2017, AFMA handed over responsibility for  calculating BBSW to the Australian Stock Exchange. The process has been  evolving and in May 2018 was confirmed in a paper ASX  BBSW Trade and Trade Reporting Guidelines.
In an ASIC media release dated Monday 21 May 2018, they  commented:
  The bank bill swap  rate (BBSW) rate is a major interest rate benchmark for the Australian dollar  and is widely referenced in many financial contracts. Previously, BBSW was  calculated from the best executable bids and offers for Prime Bank securities.  A major concern over recent years has been the low trading volumes during the  rate-set window, the period over which the BBSW is measured.
    
  The new BBSW methodology  calculates the benchmark directly from market transactions during a longer  rate-set window and involves a larger number of participants. This means that  the benchmark is anchored to real transactions at traded prices. ASX, the  administrator of BBSW, has consulted market participants on this new  methodology. In addition, the ASX has recently conducted a successful parallel  run of the new methodology against the existing method.
 
BA Deputy Governor  Guy Debelle said,
‘The new methodology strengthens BBSW by anchoring the  benchmark to a greater number of transactions. This should help to ensure that  BBSW remains robust.’
 
BBSW Waterfall