HD is back and better than ever.
Channel 9 revived its HD channel back on the 26th of Nov 2015 and Channel 10 revived its HD channel on the 2nd of March 2016. Channel 7 was also planning a HD simulcast of channel 7 in time for the Australian Open, but that didn't happen. It's also unlikely they will have it ready in time for the footy season. ABC is also planning to simulcast in HD, while SBS is planning an upgrade to its HD simulcast.
Broadcasters are also shifting to the more bandwidth efficient MPEG4 standards instead of using the ancient MPEG2 standards, though MPEG4 was announced in 1998 and implemented in the early 2000s. The new HD channels take up more or less the same bandwidth as a sdandard definition 576p MPEG2 channel, yet offering much improved definition and quality.
Both 9HD and TEN HD take advantage of MPEG-4 AVC (H.264) video codec which now offer a full 1920x1080 frame instead of 1440x1080 by MPEG2. The quality of the video is much improved and colours are much more vivid. You are also much less likely to see the picture turn to blocks during complicated scenes, as H.264 deals with complicated scenes much better than MPEG2. This was especially noticeable during fade-ins and fade-out scenes as well as fast moving scenes.
Channel 7's TV4ME, 7flix and RACING.COM have all be upgraded to MPEG4 (AVC/H.264 for video and AAC for audio), which improved the quality while decreasing bandwidth usage.
However, with these upgrades, people with older devices (with older decoders in them) will not be able to decode MPEG4 channels. Currently, people with old devices are only missing out on 7flix, as all the other MPEG4 channels are HD simulcasts of MPEG2 standard definition channels.
People who have bought Freeview certified products are guaranteed to support the MPEG4 standards. If you are able to watch 9HD and TEN HD, then your device is also MPEG4 compatible. For now, people with older devices can rest assured that the main channels (ABC1, SBS1, Seven, Nine, Ten) will continue to broadcast in the older MPEG2 standard.
As much as I don't wish for people to need to upgrade their devices, I do hope that MPEG2 transmissions will get faded out soon. MPEG2 is just getting too old and too inefficient.
Channel Bitrate Comparison
More details can be found on this page.