<apex:pageBlockSection title="Residential Postcode" columns="1" collapsible="false">
            <apex:inputText id="myField"
                            label="myField"
                            value="{!myField}"
                            size="12" maxlength="12">
                <apex:actionSupport event="onchange" reRender="sectionOne, sectionTwo" />
            </apex:inputText>
        </apex:pageBlockSection>

<apex:pageBlockSection title="Residential Postcode" columns="1" collapsible="false">
            <apex:actionSupport event="onchange" reRender="sectionOne, sectionTwo" />
            <apex:inputText id="myField"
                            label="myField"
                            value="{!myField}"
                            size="12" maxlength="12" />
            </apex:actionSupport>
        </apex:pageBlockSection>