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<?php
/**
 * WooCommerce Extra Feature
 * --------------------------
 *
 * Restrict shipping countries list
 *
 */
function woo_override_checkout_fields( $fields ) { 

  $fields['shipping']['shipping_country'] = array(
		'type'      => 'select',
		'label'     => __('My New Country List', 'woocommerce'),
		'options' 	=> array('AU' => 'Australia')
	);

	return $fields; 
} 
add_filter( 'woocommerce_checkout_fields' , 'woo_override_checkout_fields' );