Add shapefile to ggmap

Somewhat similar to the issue described here I am having troubles aligning a shapefile and ggmap object.

1. My shapefile consists of local area boundaries in Victoria, Australia, and I am trying to overlay them on top of a google map of the state (Victoria).

   The source shapefile has the following PROJ4 string (extracted from the prj file)

   [+proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs]
   

   which corresponds to EPSG:4283.

   Here's the summary of my shapefile object sp:

   > summary(sp)
   Object of class SpatialPolygonDataFrame
   Coordinates:
           min        max
   x  96.81677 159.109219
   y -43.74051  -9.142176
   Is projected: FALSE
   proj4string:
   [+proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs]
   

2. I convert the shapefile to match google's pseudo Mercator projection (at least that's what I think I'm doing)

   sp <- spTransform(sp, CRS("+proj=longlat +init=epsg:3857"))
   

   and turn sp into a fortified dataframe df.sp.

3. I then use

   map <- get_map("Victoria", zoom = 7, maptype = "terrain", source = "google")
   

   to get a google terrain map of Victoria, and (gg)plot them

   ggmap(map) + geom_polygon(data = df.sp, aes(x = long, y = lat, group = group)) + coord_equal() + theme_map()
   

It's clear from the resulting plot that shapefile coordinates and googlemap coordinates don't overlap. Am I doing something wrong with the coordinate transformation? How do I properly match shapefile and googlemap coordinates? I would appreciate any help/insight into this matter.