Mr Tanaka sailed into Sydney Harbour on Monday aboard the "Peace Boat" carrying survivors of the Hiroshima and Nagasaki bombings and descendants of those impacted by the 1950s British nuclear weapons testing at Maralinga in South Australia.

"The first thing I remember about the (Nagasaki) nuclear blast was a blinding white flash that surrounded my body," Mr Tanaka said through an interpreter.

"I tried to go upstairs but at that moment there was a huge blast."

When he regained consciousness he found himself pinned under an intact glass door and "for some miraculous reason" he was uninjured.

Three days after the blast, then 13-year-old Terumi found the bodies of his two aunts, an uncle and a cousin who lived much closer to the epicentre of the blast.

His grandfather was severely injured and died soon afterwards.

The nuclear weapons testing survivors were joined by activists in Sydney at a rally calling for all nations to sign the UN Treaty for the Prohibition of Nuclear Weapons.

Australian anti-nuclear campaigner Dave Sweeney says US President Donald Trump's rhetoric hasn't helped limit the nuclear threat posed by North Korea.

"We need to talk, we need to have strategies, engagement, diplomacy, trade, sanctions, aide, carrot and stick," Mr Sweeney told AAP on Monday.

"It is not helpful for the president of the United States to say 'my button is bigger than yours and I am prepared to use it' on Twitter."

The Treaty on the Prohibition of Nuclear Weapons opened for signature in September 2017.

Activists on the Peace Boat are calling for Japan and Australia, in particular, to reject nuclear weapons and join the new UN treaty.

The 11-storey vessel is visiting Sydney as part of its "making waves" tour which is exploring the devastating humanitarian consequences of the use and testing of nuclear weapons.