Multiple domains for a single locale
Use a Multi-Environment configuration to set the siteUrl for your two locales, en_au and en_us, independently for each domain like so:

'project.com.au' =&gt; array(
    'siteUrl' =&gt; array(
        'en_au' =&gt; 'http://project.com.au/',
        'en_us' =&gt; 'http://project.co/',
    ),
),

'project.co.nz' =&gt; array(
    'siteUrl' =&gt; array(
        'en_au' =&gt; 'http://project.co.nz/',
        'en_us' =&gt; 'http://project.co/',
    ),
),

'project.co.za' =&gt; array(
    'siteUrl' =&gt; array(
        'en_au' =&gt; 'http://project.co.za/',
        'en_us' =&gt; 'http://project.co/',
    ),
),

'project.co' =&gt; array(
    'siteUrl' =&gt; array(
        'en_au' =&gt; 'http://project.com.au/',
        'en_us' =&gt; 'http://project.co/',
    ),
),


You can also set additional environment / domain specific variables in the config file. You could set the currency or the timezone for each domain:

'project.com.au' =&gt; array(
    'currency' =&gt; 'AUD',
    'timeZone' =&gt; 'UTC+10',
    'siteUrl' =&gt; array(
        'en_au' =&gt; 'http://project.com.au/',
        'en_us' =&gt; 'http://project.co/',
    ),
),


This is how you access such a variable in your templates:

{{ craft.config.currency }}