pre_query.php

<?php
function( $query ) use ( $custom_post_type_name ) {
			if ( ! is_admin() && $query->is_main_query() ) {
				if ( ! empty( $custom_post_type_name) && $query->is_post_type_archive(  $custom_post_type_name ) ) {
					$date_sydney_now = ( new \DateTime( 'now', new \DateTimeZone( 'Australia/Sydney' ) ) )->format('Ymd');
						/** Default to upcoming events. */
						$compare = '>=';

						if ( 'true' === $_GET['archive'] ) {
							/** We need to get everything older than today. */
							$compare = '<=';
						}

						$query->set( 'posts_per_page', 5 );
						$query->set( 'meta_key', 'start_date_picker' );
						$query->set( 'orderby', 'meta_value' );
						$query->set( 'order', 'DESC' );
						$query->set(
							'meta_query',
							array(
								array(
									'key'     => 'start_date_picker',
									'value'   => $date_sydney_now,
									'compare' => $compare,
								)
							)
						);
					} 
				}
			}