This thing happened on my BMO account too.
I found $2700 be transferred out from my account and I phoned BMO on the same day, BMO told me they received an email from my email account for this transfer request,
BMO stopped the transfer when the money still on the way. If the transfer competed, they couldn't get the money back,
E-transfer only need the debit card number and not required the password if the transfer request coming from my email address which registered in BMO, The hacker stole my bank card number and hijacked my email account, there was nothing wrong with BMO,
After then I stopped all e-transfer option at all my bank accounts, replaced new bank card, changed the email password to a very complicated combination.
Good luck, hope you can get your money back.