Hi sarahtaylor93,
Nice work narrowing it down to that pay run. That one with the wrong pay date will have pushed an extra STP event through to the ATO.
First thing I would do is run your Payroll activity report for Jan to Mar and check the PAYG total against what you expect for the BAS. If that looks right in MYOB, jump into Payroll, then Single Touch Payroll reporting, then STP reports, and send an Update event so the ATO gets the corrected year-to-date figures.
Once that Update event shows as Accepted at the ATO, the PAYG they have for the quarter should line up with your MYOB reports, and you can rely on the MYOB figures for your BAS.
If it is still not matching after that, it is best to get our support team to take a closer look. You can either lodge a ticket in My Account or chat to the team via MOCA, our virtual assistant.
Regards,
Genreve