A bit of history about these twin Jetties:
I have always known these jetties as the cattle jetties but have really never knew why or the history behind this. Finding the history about the jetties was a bit elusive.
What I did find out is that the twin jetties were first built on this site in the mid 1800's for unloading of cattle to the abattoirs and stock yard on the site.
"A vessel with cattle on board lies alongside both (jetty) Ts, and discharges cattle into the water between the jetties, and they swim ashore and walk along a railed enclosure straight into the slaughter yards. This arrangement is made so that if a beast suffering from any disease it is in practically quarantine.."
Picture below showing Cattle Jetty Abbatoirs c1872 
Responsibility for the jetties was assumed by the Marine Board in 1886, "resulting in the jetties being restored with stone approaches and were about 120 feet with T ends, measuring 40 feet by 12 feet".
I suspect significant works were done to the jetties over the period of the two world wars as they were used for naval purposes. "During the late 1950's the Naval Jetty, at the old Cattle Jetties site, was reconstructed in concrete using precast elements."
At some stage I am assuming the current carpark area was also reclaimed making the jetties look shorter than they originally were.
Above quotes from "Capital Ports, A History of the Marine Board of Hobart 1858-1997", A Hudspeth and L Scripps.
Many activities of Australia's longest running annual regatta(Royal Hobart Regatta) have been focused around these jetties. This year (2013) will be the 175th year of the regatta.
Below picture showing the 1936 Hobart Regatta and cattle jetties 
Please don't get this cache if muggles are fishing on the end of the jetty.