Hides all but the Australian states in the state field for ONTRApages. Only works for chrome and firefox, ie doesn't support hiding options via css. Just drop this gist into the custom header code textarea in the settings tab

ONTRApages - only show AU states in the state field

<style>
    /* hide all options in the state field */
    select[name="state"] option {
        
        display: none;
    }
    
    /* target the states we want to show via the value attribute */
    select[name="state"] option[value="ACT"],
    select[name="state"] option[value="NSW"],
    select[name="state"] option[value="VIC"],
    select[name="state"] option[value="QLD"],
    select[name="state"] option[value="AU_NT"],
    select[name="state"] option[value="AU_WA"],
    select[name="state"] option[value="SA"],
    select[name="state"] option[value="TAS"],
    select[name="state"] option[value=""],
    select[name="state"] option[value="_NOTLISTED_"],
    select[name="state"] option[value="ACT"] {
        
        display: block;
    }
</style>

Kevin Eales Monroe mentioned to me that a similar block of CSS is needed to set the states for the billing country as well:

<style>
    /* hide all options in the state field */
    select[name="billing_state"] option {
        display: none;
    }
    /* target the states we want to show via the value attribute */
    select[name="billing_state"] option[value="ACT"],
    select[name="billing_state"] option[value="NSW"],
    select[name="billing_state"] option[value="VIC"],
    select[name="billing_state"] option[value="QLD"],
    select[name="billing_state"] option[value="AU_NT"],
    select[name="billing_state"] option[value="AU_WA"],
    select[name="billing_state"] option[value="SA"],
    select[name="billing_state"] option[value="TAS"],
    select[name="billing_state"] option[value=""],
    select[name="billing_state"] option[value="_NOTLISTED_"],
    select[name="billing_state"] option[value="ACT"] {
        display: block;
    }
</style>