Seventy-five years ago, under the cover of darkness, a group of Japanese midget submarines secretly cruised into Sydney harbour and launched an attack that would bring the Second World War to the city's doorstep.

Twenty-one Australian and British sailors were killed when a converted ferry, the HMAS Kuttabul, was sunk by a torpedo in the surprise naval offensive that shocked Sydney.

Left shattered was the belief held by many Sydneysiders they were safe and protected from the conflict being played out across other parts of the world.

Cecilia Southwick who was six at the time, today took a ferry ride across the harbour to retrace in her experience of the fateful event.

She remembered the explosions that shattered what was otherwise to her and the rest of the city, a peaceful night.

"We were at home on the water at Greenwich.

"I remember the flood lights ... we thought there was an invasion," she said.

"I think my parents were very worried about our grandparents who lived further around on the waterfront too."

Iris Moore, who was 12 at the time of the attack, said she remembered sitting at her kitchen table when she heard the first sirens ringing out at about 7:00pm.

"What's happening What's happening?" she remembered asking.

"When we looked out [the window] the whole street was out, everyone was wondering what was happening.

"It went for quite a while ... the noises were massive."

Iris Moore, who was aged 12, feared there was an invasion underway. ( ABC News: Sarah Hawke )

Torpedo launched by rogue sub

The three two-man submarines had been deployed off the Sydney coast - their aim was simple - to sink the USS Chicago and any other allied warships inside the harbour.

Two of the vessels were detected and failed to make a difference with crews detonating devices and taking their own lives.

A full-scale search of the harbour found the third submarine, which evaded fire and launched two torpedos at the USS Chicago, its prime target.

The torpedos missed with one of the explosions splitting another boat the HMAS Kuttabul in half - leaving 19 Australians and two British officers dead, and another 10 injured.

Keiko Tamura, a visiting fellow from Japan at the ANU's school of culture history and language, said the city was left shaken for days to come.

"The fact that the Japanese could attack and penetrate into the harbour and also attack the military target, it was a big shock for Sydney, " Dr Tamura said.

The wreck of HMAS Kuttabul sits in the waters of Sydney Harbour at Gardens Point after the attack. ( Australian War Memorial/ID: 305105 )

Further attacks were a real concern

Sydneysiders remained fearful of further attacks with Japanese vessels stationed along the coastline and false sightings of submarines added further fear to the possibility of an invasion.

"They were still wondering if there were other submarines still lurking in the harbour," Dr Tamura said.

"Those submarines were so difficult to detect. It was quite a worrying and scary experience for people in Sydney."

Two of the submarines were recovered shortly after the attack and are now at the Australian War Memorial.

In a move criticised by the public, four of the Japanese sailors who were recovered, were given funerals with full military honours.

It was a move leaders in Australia hoped would improve conditions for Australian prisoners held by the Japanese overseas.