Can’t load WordPress admin dashboard eg. mydomain.com.au/wp-admin before or after trying to login?
Its likely to be related with one of your plugins.
We don’t know which plugin is causing the problem. One way to try and find out is manually deactivating each plugin one by one by renaming each plugin folder one by one by using ftp or cPanel File Manager wp-content/plugins/ and test each time for the dashboard to load.
Alternatively you can deactivate all plugins at once by renaming the plugins folder to something else using ftp or cPanel file manager, and then login and reload the dashboard and website, then change the plugins folder back to ‘plugins’ again using ftp or cPanel file manager, then go to plugins page and reactivate each plugin one by one and reload the dashboard each time to find which plugin is causing the problem.
If that does not resolve the issue, try switching to a default WordPress theme like “Twenty Nineteen” to rule out a theme specific issue. If you don’t have access to your Dashboard, access your server via FTP, or cPanel file manager, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule out a theme specific issue. You can download the default Twenty Nineteen from the wordpress.org site if you don’t have it already.
There a many different reasons a plugin can cause trouble other than when its first installed.
Same goes for themes too.
Best thing is to keep plugins and themes updated and remove themes and plugins you don’t use or need or appear to be causing a problem. This will minimise the possibility of problems and of your site being hacked.
One particular plugin that has been causing this problem lately is the new “woocommerce-admin” plugin. If you have that, try changing its folder name first to something else then try the login.