Rules before 1 January 2020:

• Before 1 January 2020 you can ELECT to continue paying SG based on your employee's pre-sacrifice salary (or you may be contractually oblidged, under the agreement, to pay both the super guarantee and salary sacrifice).
  • your minimum SG payments are calculated on the reduced salary (after deducting salary sacrifice)
  • the salary-sacrificed amount counts towards super guarantee(do check Fair Gov awards to confirm your industry's rules)
    • e.g. Employee earns gross $1000 + salary sacrifice $100 (ignore tax). Thus his net gross (before tax) is $900
      SG calculation of 9.5% = $95
      Since Salary sacrifice $100 > super guarantee $95, the ATO will accept that the employer fulfilled their staff's super obligation by just paying the salary sacrifice. Thus saving the employer $95 (instead of paying $195).
    • Do check the Fair Gov awards to see if you are still oblidged to pay.

Current rules:

• After this date, salary sacrifice will not reduce the your super guarantee obligations, regardless of the amount your employee elects to salary sacrifice.
  This means the salary sacrificed amount does not count towards your super guarantee (SG) obligations.
  • A further change is that the super guarantee will be 9.5% of the employee's ordinary time earnings(OTE) 'base'. The base is the sum of:
    • the employee's OTE, and
    • any amounts which would have been OTE, had they not been salary sacrificed into a complying super fund or Retirement Savings Account.
• Maximum contribution base - you don't have to pay SG for employee's earnings above a certain limit. 
• If you go over the concessional cap, the excess is included in your assessable income for tax return.

Reference

• Super guarantee payments Last modified: 19 Feb 2018    QC 17235