At about 11:36 AEST today (9.Oct.2006) the Democratic People's Republic of Korea (North Korea's official name) detonated a Nuclear weapon in an underground test. There should be minimal or no short term radioactive leakage.
It was detected by South Korea's Institute of Geoscience and Mineral Resources as a tremor of a magnitude 3.58 to 3.7, and the US Geological Survey detected it as a tremor of a magnitude 4.2
The location of the blast was North-East of the country, 70 km North of Kimchaek, North Korea. 90 km South West of Chongjin, North Korea, and 180 km south of Yanji, Jilin, China.
A Seismic event of 4.5 on the Richter scale according to this University of Nevada guide would equate to a blast yield of about 5.1 Kiloton, which makes it about a quarter the size of the Atomic bomb dropped on Nagasaki in WWII. Of course this is a rough estimation of the blast yield of the North Korean Nuclear device, and not a definitive indication of typical nuclear warheads in the North Korean arsenal.
Interestingly, China who have long been seen as Pyongyang's most important ally and the only country with enough leverage to influence Kim Jong Il's behavior have denounced the test as "brazen" in unusually strong language, and demanded Pyongyang stop any action that could worsen the situation.
Missiles which North Korea were testing earlier this year include the Taepodong-2 which is estimated to have a range of 6000 km. Enough to reach Brisbane.
Hope you find this reference material useful as something to ponder on, and please remember that Brisbane has been in range of a lots of nuclear weapons for a long time, so the are no new concerns in this regard.