Generally, you need a valid adjustment note before you can make a decreasing adjustment, unless the adjustment is for GST of $75 or less.
For example, an adjustment event that discounts the payment for the original sale of the goods or services by $660 results in a decreasing adjustment (GST liability) of $60. As this is below the $75 threshold there is no need to issue an adjustment note. It is not required for attribution purposes either.
There may be other circumstances where you do not have to hold an adjustment note.
If you need to report an adjustment but you don't have the required adjustment note when you lodge your activity statement, you report the adjustment in the first activity statement you lodge after receiving the adjustment note.
You can't make a decreasing adjustment for a transaction that didn't include GST, even if you have a document from a supplier that states it is an adjustment note.
See also
- Accounting for GST in your business
- GSTR 2013/2 Goods and services tax: adjustment notes