All non-www pages redirect to the home page

All non-www pages of my WordPress multi-site redirect to the www version of the home page. I’ve tested multiple WordPress themes to rule out my theme and the same issue occurs. It also occurs on other sites within the multisite installation.

I have found a similar issue, but no solution that works for me:
https://wordpress.org/support/topic/missing-www-from-our-url?replies=16

Here is my WordPress .htaccess file:

# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]
RewriteRule  ^[_0-9a-zA-Z-]+/(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
# END WordPress

I also test the following after RewriteRule ^index\.php$ – [L] to no avail:

# BEGIN NON-WWW REDIRECT TO WWW
RewriteCond %{HTTP_HOST} ^domain.com [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,NC]
# END NON-WWW REDIRECT TO WWW

These are unusual settings at the bottom of this existing install that I don’t have on other standard WP installs:

define('MULTISITE', true);
define( 'SUBDOMAIN_INSTALL', false);
$base = '/';
define('DOMAIN_CURRENT_SITE', 'www.domain.com.au');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);
if ( !defined('ABSPATH') )
	define('ABSPATH', dirname(__FILE__) . '/');
	require_once(ABSPATH . 'wp-settings.php');

The site is WordPress 4. I thought WordPress itself should automatically do the non-www redirect out of the box if it has the domain setup. Do you have ideas as to what is causing the issue or ways to diagnose?