如果去年2017 9月份的tax invoice呈上去给kastam了，2018今天才看到有错误（数目少了一个零）
The missing number is it tax invoice or the amount.
tax invoice里面的数目写少一个零 （10,000.00 – 大少一个零 1,000.00)
然后accoutant根据我的tax invoice数目呈报去kastam，结果account进入数有很大分别 才发觉到错了
You can issue a Debit Note / Advice to the customer and explain being amount under-charged by referring that particular Tax Invoice which you have issued.
Penalty or not is up to the GST Officer-in-charge to decide.
btw, 谢谢你的提议 ^^
Focus on info & knowledge on GST - See guide