I rang ASB call centre staff, she told me that at the time I adjusted my limit to $3000, my balance was over $6000. I asked her why it didn’t warn me, if my balance is over $6000 then it should not let me adjusted lower than my balance and then charged me “Over Limit Fee”, she told me “You should read the term and condition” blah blah blah. I told her that the Term and Condition only mention about charge for over 10% but not anything about the allowing you to adjust limit lower than your balance and charge you for "Over Limit Fee". Although she reversed the fee for me but I hope she can learn the system well before arguing with her customer. She is very unprofessional.
I spoke to another banker in ASB, I was advised that usually I can’t adjust the limit any lower than my balance. This makes sense.