I've come across an excellent post on the Bizify website which explains commonly used business registration acronyms.  

Head across to Bizify's 'What is ABN, ACN AUSkey? - World of business registration acronyms' by Ritesh Anan.  

The article was posted a couple of years ago.  It includes these acronyms and is relevant today:

ABN            Australian Business Number

ACN            Australian Company Number

ASIC            Australian Securities and Investment Commission

ATO             Australian Taxation Office

AUSkey       Single key access to government online services like ASIC Connect,
                    business name search etc.

GST             Goods and Service Tax

PAYG           Pay As You Go

PAYGI          Pay As You Go Installment

PAYGW        Pay As You Go Withholding.

TFN              Tax Filing Number