The W1 is the total gross wages/salary payments from which you are usually required to withhold amounts
For more information click on the following ATO link https://www.ato.gov.au/Business/Business-activity-statements-(BAS)/In-detail/Instructions/PAYG-withholding---how-to-complete-your-activity-statement-labels/?page=3#W1Totalsalarywagesandotherpayments
For example:


iv. Wages Account - this is used to indicate (expense type) accounts to be included in gross wages figure for BAS return