Yes, that is exactly the situation.
Electric power is being extracted from northern Manitoba for the benefit of southerners in southern Manitoba, and some of the power is sold to other provinces and American states. The benefits go to Manitobans and largely to Manitoba Hydro itself and the provincial government, which gets a lot of money from Manitoba Hydro, but the benefits don't go to northern first nations.
In recent years, we've seen some partnership agreements between Manitoba Hydro and first nations, which make it look as if there's a broader consent and a partnership happening, but these deals are very poor deals for first nations. They still aren't getting a lot of benefits. They're not getting even the benefits they were promised. Still, the harms continue.
There was a treaty made in 1977, the Northern Flood Agreement. It's been nullified, first by just delaying and litigating everything, then eventually by passing new agreements.