<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript">
$(document).ready(function() {
$("#suburb").blur(function() {
if ($(this).val() != '')
$("#post_code").attr("disabled", "disabled");
else
$("#post_code").removeAttr("disabled");
});
$("#post_code").blur(function() {
if ($(this).val() != '')
$("#suburb").attr("disabled", "disabled");
else
$("#suburb").removeAttr("disabled");
});
});
</script>
You'll also need to add a value attribute to the first option under your select element:
<option value=""></option>