Hi AG23,
The old pay run "not sent status" will remain on the STP screen as there is no option to remove it. The best course of action is to leave the "not sent" status as it is and ensure that your payroll reports are accurate. Sending an update event is usually required if there are discrepancies in your payroll reports and if you update your employees payroll information with ATO.
Regarding automatic sending of events, typically, once you process and complete the steps on processing pay runs, those pay events are automatically sent to the ATO, and you'll see them reflected in your STP reporting.
Cheers,
Princess