As I said, in your case you'll have to send 3 packages to Australia Post and the fee will be the same regarding the state of your "Group package" option.
But if you want to restrict the use of your Australia post shipping method just for when you'll have to send 1 package, the solution would be to edit the code of your Australia Post shipping plugin and to add this code :
$order_weight = 0;
$order_volume = 0;
$order_width = 0;
$order_height = 0;
$order_length = 0;
foreach($order->products as $k => $product){
if(!empty($product->cart_product_quantity) && !empty($product->product_weight)){
if(!isset($product->product_weight_unit_orig))
{
if(isset($product->product_weight_orig))
$order_weight+=(int)$weightClass->convert($product->product_weight_orig,$product->product_weight_unit,'g') * $product->cart_product_quantity;
else
$order_weight+=(int)$weightClass->convert($product->product_weight,'','g') * $product->cart_product_quantity;
if(!isset($product->product_dimension_unit))$product->product_dimension_unit = 'm';
$order_volume+=(int)$volumeClass->convert($product->product_length*$product->product_width*$product->product_height,$product->product_dimension_unit,'mm')* $product->cart_product_quantity;
$order_width+=(int)$volumeClass->convert($product->product_width,$product->product_dimension_unit,'mm','dimension')* $product->cart_product_quantity;
$order_height+=(int)$volumeClass->convert($product->product_height,$product->product_dimension_unit,'mm','dimension')* $product->cart_product_quantity;
$order_length+=(int)$volumeClass->convert($product->product_length,$product->product_dimension_unit,'mm','dimension')* $product->cart_product_quantity;
}
else
{
if(!isset($product->product_dimension_unit_orig)){
$product->product_dimension_unit_orig = $product->product_dimension_unit;
}
if(isset($product->product_weight_orig))
$order_weight+=(int)$weightClass->convert($product->product_weight_orig,$product->product_weight_unit_orig,'g')* $product->cart_product_quantity;
else
$order_weight+=(int)$weightClass->convert($product->product_weight,$product->product_weight_unit,'g')* $product->cart_product_quantity;
$order_volume+=(int)$volumeClass->convert($product->product_length*$product->product_width*$product->product_height,$product->product_dimension_unit_orig,'mm')* $product->cart_product_quantity;
$order_width+=(int)$volumeClass->convert($product->product_width,$product->product_dimension_unit_orig,'mm','dimension')* $product->cart_product_quantity;
$order_height+=(int)$volumeClass->convert($product->product_height,$product->product_dimension_unit_orig,'mm','dimension')* $product->cart_product_quantity;
$order_length+=(int)$volumeClass->convert($product->product_length,$product->product_dimension_unit_orig,'mm','dimension')* $product->cart_product_quantity;
}
}
}
if($order_length > 1050 || $order_width > 350 || $order_height > 350 ){
return false;
}