So I got the Witcher 3 and started downloading all 7 setups from the browser. Then an error would pop up something like Checksum.
This happens around 2 minutes into the setup where it would popup a message then soon after the error that shuts the setup down.
The website doesn't really help. Just says to try galaxy which forces me to restart downloading everything again and my files don't seem corrupt
If it helps all the files from 2-5 are 4,147,200 kb. Setup 1 is 4,044,800 and the setup 6 is 2,354,533

Any help would be appreciated :)

Unfortunately a checksum error does mean that your files are corrupt. Just because they seem right doesn't mean that some bits were not changed.

Use Galaxy, or if you don't want that any download manager.

When you launched the setup (installer), did you go into its options to enable integrity check? Make sure it is enabled, because then it will check the integrity of your installer files (the .bin files), before it proceeds with the actual installation.

I presume it then tells which file is corrupt, which you need to redownload.

(I'm surprised though if the integrity check isn't enabled by default, I don't remember if it is. It would make lots of sense to be enabled.)

If you skip the integrity check, then you'll probably end up with a partial installation. This happened to me once when I also installed a GOG game with a corrupted installer, and the installation would abort midway when it detected too late that one of the files is not ok. So I ended up with a partial installation, which I couldn't even uninstall.

The fix was simple though: when I had the working uncorrupted installer, I merely reinstalled the game over the failed installation. It worked ok, and could also be uninstalled properly.

Note that there are three optional ways to download the installer files:

1. Direct web links on the GOG site.
2. With the old GOG Downloader client.
3. With Galaxy (going to the extras and downloading the "backup version" there.)
(4. You can also download the Galaxy version with the Galaxy client, but this is irrelevant for your case.)

I think the latter two will try to download everything again, so you either have to abort the download of those parts which you already have uncorrupted, or download only the relevant .bin file with the direct web link. Are you able to provide e.g. md5sum for your installer files? Then me or someone else could provide the correct checksums, to identify right away which file is corrupted or incomplete.

Unfortunately I don't have the installer files on this PC, so I can't provide the correct md5sums right now...

Apparently there is a checksum utility already at least in Windows 7, you'll need to use it through the command line prompt: Oh, but is the Australian version somehow different though? Censored? Which would mean you'd probably have different md5 checksums anyway than I would for the English version?

The Witcher 3 FAQ

Copy the folder from a working installation of a friend on your harddrive, start thewitcher.exe - have fun.

Ok thanks, there are the md5 checksums. (I presume they are the same for all versions)