Why is the GST a cent different from 1/11?

Because Sports Club OS rounds the GST once, over the whole invoice, as the ATO's total invoice rule allows, and then shares that amount across the lines to the cent. Working each line's 1/11 out on its own and adding them up can come to a cent more or less, so one line takes the odd cent.

An example

An invoice for a ball machine hire at $15.00 and a racquet restring at $35.00, both taxable, totals $50.00.

• The invoice's GST is 1/11 of $50.00, which is $4.545…, rounded once: $4.55.
• On their own, the lines' GST would be $1.36 and $3.18, which add up to $4.54.
• The missing cent goes on the largest line. The invoice shows $1.36 and $3.19, and Includes GST $4.55.

The same happens with equal lines: on an invoice with three $900.00 joining fees, each line's 1/11 rounds to $81.82, but the invoice's GST is $245.45, so one line shows $81.81 and the other two $81.82.

Where else a cent can move

• Payments. A payment's share of an invoice's GST is worked out on the running total, so a part payment can carry a cent more or less than 1/11 of itself. The payments on a fully paid invoice always add up to the invoice's GST exactly.
• The BAS worksheet. 1A adds up the GST each invoice and payment actually carries, so it can differ by a few cents from 1/11 of G1.
• Estimated GST. Money imported from the system the club used before, and older till sales, carry no GST of their own; theirs is taken as 1/11 and marked estimated.
• An older invoice keeps the GST it was raised with. Sports Club OS never works an invoice's GST out again, so an invoice raised before GST codes existed can be split slightly differently.

None of this changes what anyone pays: the invoice total always includes its GST. See How GST is worked out, and how prices are entered.