sorry to be graphic , again. Dead bodies manufacture gas, the skin breaks down under water, what isn't tied down floats up, this happens in water that is anything more than icy, anything over that which is about to become ice assists the natural process of breakdown of the epidermis, and underlying tendons, in other words, bits become detached and float free. A body does not retain it's integrity underwater, there are a lot of factors that go towards the dissolvement, even in very cold water, but AU Feb water, it's a very quick process..
For example, in the Murray River, about 300 klms north of Ballarat, ( hotter, so to speak ) in summer every year someone drowns, often early in Dec, but always a few in Jan and Feb, and from long practice the local police at Albury, or Corowa,, or Yarrawonga, wait about 2 or 3 days, because the body will float up, even if it has been caught under a snag, ie tied down by branches ....
This is old news, .. shallow dams around Ballarat, ( and urban dams, like the phone dam, and the ones along the same road, they are not especially deep, ) warm up fast, and cool down slow. ... a body in one of those dams would be visible by the Wednesday, or Thursday , after she went missing on the Sunday. Unless he put it in one of the big river reservoirs .. even Lake Wendouree is very shallow, it is not a deep lake.