Optus no longer Foxtel so you will have to get it from either Telstra or Foxtel directly. It will almost certainly be a satellite service not a cable one.
As for existing boxes, Optus will send you a satchel(s) to return them.
Thanks for the information much appreciated - I spoke to someone at Optus and looked at the Foxtel deals so will be taking up the iQ box options at this stage with Foxtel but keep my NBN with Optus.
One quick question I had before I switched over is, we've got an IQ box (with HD) downstairs. We know we have to send the boxes back to Optus, but since it still has unwatched content on the IQ box - can we keep that until we have watched the content and then send it back to Optus (since we technically don't need a connection to watch that as it is on the box).