Manitoba's Emergency Measures Organization said Friday that the faulty gauge, located on a tributary of the Assiniboine, had underestimated the river's flow by about 2,500 cubic feet per second.
Accurate readings now show the Assiniboine will peak about 1.5 feet higher than originally expected.
The top of Brandon's dike is 1,184 feet above sea level; the waters are expected to peak at about 1,183 feet above sea level.
Flood fighters usually build dikes two feet higher than necessary to accommodate unusual flows.
The Assiniboine has already surpassed its previous record set in 1923, and the rainy forecast is cause for concern.
A couple of inches would get us scrambling, Brian Kayes, an emergency measures co-ordinator for the Brandon area, said Friday.
Unfortunately, southern Manitoba can expect a lot more than a shower this coming week.
Brian Dillon, a meteorologist at The Weather Network, says back-to-back lows are poised to sweep over the region, dumping up to 55 mm of rain on areas where flood concerns are most heightened, including Brandon.
The first trough will arrive Sunday evening, bringing with it scattered showers and the chance of isolated thunderstorms through Monday. The second, more powerful low will hit on Tuesday and take days to depart. That means the south, especially the border region, is in for a soaking.
We're looking at 15 to 25 mm of rain today and tomorrow, and an additional 20 to 30 mm from Tuesday to Thursday. Closer to the border, you're looking at an additional 30 to 40 mm, says Dillon.
Both the Assiniboine and Red Rivers are cresting and officials say so far, the flood protections set up around the provincial capital are working. However, some communities, especially First Nations, haven't been so fortunate.
With files from the Canadian Press and CBC Manitoba