Good day eh!!!
Last time I checked, no such thing as a "command" instrument rating in Canada. Just................an instrument rating. You've either got one or you don't.
One of our guys (Ozzie CPLH only) was sent to Canada (Pro IFR) to get him an IF rating. When he got there, Transport Canada said he FIRST needed to obtain a Canadian CPLH!!!! So, after much heartache and jumping through hoops, he got his Canuck IF rating AND a Canadian CPLH. Go figure.
I'm gonna go out on a limb here and speculate that since you've got an Ozzie ATPLH, you should be in much bette shape than our chap was when he went over.
For what it's worth, I did my helicopter IF rating back in the winter of '92 at Helicollege (Lyle Watts is still there I think) and it was great. Kicked arse on the Transport Canada exam and a seriously good ride. If you can come to grips with Lyle's inability to be on time for a flight or ground school, you'll do ok. Costs weren't that bad cuz I did my homework on any hours I had which I could apply to the minimum times required. In the end it worked out quite well cost wise.
Time wise it took about 8 weeks total, but I wasn't in a rush or on the clock so..................
Hope this helps!!!