I've never heard any Canadian use the expression, "fair dinkum," and I'm Canadian and have traveled all over Canada. That person was either Australian or was confusing Australuan and British phrases and was trying to fit in.
To me, a toilet is the receptacle for one's bodily waste, not the room. And it seems odd to criticize the use of "washroom" when lavatory means washroom. (The root, lav, Is from a Latin word for washing.)
As a Canadian of a certain age, I have seen our language become Americanized, mainly through constant exposure to US media, but also due to spell checkers designed in the US. Thus, chips have become fries, chesterfields have become sofas, thongs have become flip flops, Hallowe'en Apples has become Trick or Treat and the u is disappearing from many -our spellings, although I still cling to u and always will.
I resent the homogenization of North American English, and I enjoy the differences when I travel or meet people from overseas, and I think of their way of speaking as interesting rather than wrong. Vive la differerence!