you must add a condition statement to the dropdown. I suggest that you store the list of suburbs in the db or in a separate array.
first pull data from the db
let's assume all the suburb names are stored in $suburb_array and the current suburb name is $suburb
<select name="suburb">
<?
foreach ($suburb_array as $value) {
echo '<option'; if ($value == $suburb) {echo ' selected';} echo ">$value";
}
?>
</select>