Thai bank fees charged when CDN ATM card used (Bangkok)

For every 20,000 Thai baht, I have discovered that my Siam Commercial Bank charges my Canadian bank account fees ranging from 6.5% - 8%! e.g. I withdrew 20,000฿ yesterday....conversion rate given by my bank was such that this should have amounted to $737.16Cdn. The amount actually debited from my account was $783.00 Cdn. Since my bank does not charge any fees for my ATM withdrawals (my bank advised that this was the case with my type of account), this means that a total of 1,243.62฿ was scooped up by Thai banking fees.....1,043.62฿ over and above the 200฿ ATM fee shown at machine!!
Anyone aware of this? Anyone aware of how to avoid this over Protected content surcharge on a 20,000฿ withdrawal? And by the way, this has over the past several months fluctuated from this current surcharge of just over 6% up to as high as 8%!! Any advice most welcome!!