Filtering a WP Query result

I solved my problem re-reading ACF’s custom fields documentation.

Instead of meta_query, I should’ve used meta_key.

The correct argument for a custom field is:

// args
$args = array(
    'numberposts'   => -1,
    'post_type'     => 'event',
    'meta_key'      => 'location',
    'meta_value'    => 'Melbourne'
);