An Astute class submarine with detachable pod. In this case dry-wet cell for divers (not a minisub itself - see The Sun). Other pods could take a mini-sub, diver delivery vehicle, LDUUVs or even missiles ejected horizontally.
Astute class submarine inside (Courtesy UK Daily Mail).
Some 2012 or still continuing(?) Astute class submarine problems, including noise (Courtesy 2012 Daily Mail report).
"Nicky" in comments June 15, 2015 raised the issues of Australia perhaps buying the UK newly built Astute class SSNs or non-Soryu SSKs.
It looks like the UK Astutes have not yet become fully mature and efficient submarines. That even the 2nd in class HMS Ambush was almost a decade from being laid down (October 2003) to commissioning (March 2013) suggests program problems. See problems with the Astutes. UK production of Astutes may be as delayed and overbudget as would be achieved if a sub were built in Australia.
Australia buying any nuclear submarine is unlikely due to major domestic and regional political issues, cost and basing issues.
Problems with the UK Astutes include or included "As of March 2008 the programme was 48 per cent (or £1.2 billion) over-budget and 47 months late. Further delays due to a range of technical and programme issues brought the programme to a position of 57 months late and 53 per cent (or £1.35 billion) over-budget by November 2009, with a forecast cost of £3.9 billion for the first three Astute boats.”
Such problems are major for the UK Navy even when operating its Astutes right near UK repair facilities. If any Australian Astutes experienced similar problems then a 20,000 nautical mile (80 day) round trip to-from UK shipyards would be crippling.
I'm also not sure whether the US would allow Astutes to be marketed in competition with US Virginias given all the US technology transfers to the UK nuclear submarine programs over the years including nuclear weapons and reactors.
No French Barracuda class SSNs have been launched. They suffer the same relatively distant repair facility issues with the added problem of needing "refuelling and complex overhauls (RCOHs)" every 10 years.
Regarding US Virginias and possibly Los Angeles class SSNs the US made Ambassadorial level soundings to Australia around February 2012.
If Australia did go the nuclear submarine route then buying US makes more sense. Buying Virginias would yield Pacific alliance benefits, constant interoperability, even more commonality in common combat systems and relatively close repair facilities at Guam (my next article), Pearl Harbour, Diego Garcia and other US Indian Ocean naval bases.
Other SSKs are being looked at under Australia competitive evaluation process (mainly the Soryu SMX Ocean (conventional Barracuda), TKMS-HDW 216, but also some information sought on HDW 209, HDW Dolphin 2 and DCNS Scorpene). Major program problems with S80 (Isaac Peral class) have eliminated it.