Okay, got it so that if the weight is over 20kgs, it will used the fixed prices, however don't know how to get it to use the second value instead of the first...

Code:
if ($sweight > 20000 & ($dest_country == "AU")) {
    $methods[] = array( 'id' => "ERROR",  'title' => 'Using Fixed Rate (Australia)' ,'cost' => MODULE_SHIPPING_AUSTPOST_COST_ON_ERROR ) ;
    $this->quotes['methods'] = $methods;   // set it
return $this->quotes; }  // exceeds AustPost maximum weight

if ($sweight > 20000 & ($dest_country <> "AU")) {
    $methods[] = array( 'id' => "ERROR",  'title' => 'Using Fixed Rate (International)' ,'cost' => MODULE_SHIPPING_AUSTPOST_COST_ON_ERROR ) ;
    $this->quotes['methods'] = $methods;   // set it
return $this->quotes; }  // exceeds AustPost maximum weight
Code:
$db->Execute("insert into " . TABLE_CONFIGURATION . " (configuration_title, configuration_key, configuration_value, configuration_description, configuration_group_id, sort_order, date_added) VALUES ('<hr>Fixed Cost', 'MODULE_SHIPPING_AUSTPOST_COST_ON_ERROR', '25.00,99.99', 'These rates will be used if there is an error in calucating the cost. The First value is for Australian wide. The second value for Overseas delivery.', '6', '27', now())");