gravity-au.php

<?php

add_filter( 'gform_address_types', 'your_plugin_slug_australian_address' );
/**
 * Set Gravity Forms custom addresses for Australia.
 *
 * @since 1.0.0
 *
 * @param  array $address_types Existing address formats.
 * @param  int   $form_id       Form ID.
 *
 * @return array Amended address formats.
 */
function your_plugin_slug_australian_address( array $address_types ) {
	$address_types['australia'] = array(
		'label'       => __( 'Australia', 'your-plugin-slug' ), //labels the dropdown
		'country'     => __( 'Australia', 'your-plugin-slug' ), //sets Australia as default country
		'zip_label'   => __( 'Post Code', 'your-plugin-slug' ),
		'state_label' => __( 'State', 'your-plugin-slug' ),
		'states'      => array( 
			'', 
			__( 'Australian Capital Territory', 'your-plugin-slug' ),
			__( 'New South Wales', 'your-plugin-slug' ),
			__( 'Northern Territory', 'your-plugin-slug' ),
			__( 'Queensland', 'your-plugin-slug' ),
			__( 'South Australia', 'your-plugin-slug' ),
			__( 'Tasmania', 'your-plugin-slug' ),
			__( 'Victoria', 'your-plugin-slug' ),
			__( 'Western Australia', 'your-plugin-slug' ),
		),
	);

	return $address_types;
}

The slicker, tidier version of my older code with significant input from Gary Jones (kudos).