The link I sent you hopefully shows pack liners (90 Litres specifically) and these go inside your bag. You then pack your gear inside this and roll the top over a few times and secure it in your bag.  The roll top ensures the whole thing is entirely waterproof so you can throw it down the river ahead of you if you like.

Exterior bags like pack covers, which I assume you're referring to, just cover the outside of the main area of your pack leaving space for the harness so you can still carry it.  This means that water can creep in from the harness and wind increases this risk so if you need fully waterproof, a liner is the best way to go.

The other way of doing this though could be utilising several smaller dry bags (which are small waterproof pack liners basically) to pack all your gear into and then organising these into a bag which then doesn't necessarily need further waterproofing as all your individual items are already secure.  The slight downside of this however, is that often these bags are round and so don't fit super well without losing space between them.  So I guess it depends on what you think best suits your requirements.

G'day Judy,

I can understand the confusion, sorry about that, there's a bit of technical talk involved here. It is correct that the material is waterproof as stated but the bag itself as a whole is not as the zippers are not sealed.  This means that rain could come in from this point or from seams eventually as these are also not sealed.

Having said that, the bag is very durable and would withstand a slight amount of rain however if you have a proper downpour or rain combined with blowing winds, the best way to waterproof any bag entirely is to use a pack liner which is literally a bag inside your bag to protect everything.