existing staff cannot cope...

Does the government knows the issue in practical terms? In the motor trade, parts as required to repair any vehicle are likely outsourced as the needs arise. So when the items are acquired and listed in the invoice as GST inclusive, the lazy clerk too busy may just take the inclusive price and add a margin for handling charges and bill the consumer twice for GST. It would be time consuming to charge the items minus GST as it is computed for each items. Each calculation may take a few minutes and if there are 1,000 such items, a day work is not enough when the bill has to go out to the consumers. The same company cannot afford a few more staff when an 8 hours work per day is only 480 minutes. So such companies would face penalties.