add_filter( 'woocommerce_shipping_australia_post_tax_rate' , 'woocommerce_shipping_australia_post_custom_tax_rate' );

/**
 * Adjust tax rate
 *
 * @access      public
 * @since       1.0 
 * @return      void
*/
function woocommerce_shipping_australia_post_custom_tax_rate() {
	
	return '0.09090909';
	
}