PAYG instalments are payments towards income tax on business and investment income. They are separate from GST and from PAYG withholding on employee wages.
Your notice shows either an ATO instalment amount or an instalment rate. Under the rate method, instalment income generally includes gross business and investment income, excluding GST; it is not the same as taxable profit after expenses.
If your expected tax changes, discuss a variation with us before lodging it. Keep the forecast and calculation. An estimate that is too low can lead to interest. Check income tax, GST and payroll liabilities separately when planning cash reserves.