You can use meta query in wp query to get required posts
$args = array(
'meta_query' => array(
'relation' => 'OR',
array(
'key' => 'city',
'value' => 'Melbourne',
'compare' => 'NOT LIKE'
),
array(
'key' => 'city',
'value' => 'Melbourne',
'compare' => 'NOT EXISTS'
)
)
);
$query = new WP_Query( $args );