Set a default state for the State Dropdowns Add On.

my_pmprosd_default_state.php

<?php
/**
 * Set a default Mailing Address state (State Dropdowns Add On)
 *
 * Setting a default country: 
 * https://github.com/strangerstudios/pmpro-snippets-library/blob/dev/checkout/set-pmpro-default-country.php
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmprosd_default_state() {
	// Bail if State Dropdowns is not active.
	if ( ! function_exists( 'pmprosd_states' ) ) {
		return;
	}

	// Bail if not on the checkout page.
	if ( ! pmpro_is_checkout() ) {
		return;
	}

	// Set state to Queensland on page load if country is Australia and a state is not already set.
	?>
	<script>
		jQuery( window ).on( 'load', function () {
			var state = jQuery( '#pmpro_sstate' ).val();
			if ( ! state ) {
				var country = jQuery( '#pmpro_scountry' ).val();
				if ( country === 'AU' ) {
					jQuery( '#pmpro_sstate' ).val( 'QLD' );
				}
			}
		} );
	</script>
	<?php
}
add_action( 'wp_footer', 'my_pmprosd_default_state' );

Setting a default country: https://github.com/strangerstudios/pmpro-snippets-library/blob/dev/checkout/set-pmpro-default-country.php