"If this is war, God help us."

So said 20-year-old Mervyn Lloyd Ey, who was stationed with his unit in Darwin 75 years ago today, building barbed wire defences north of the city on February 19, 1942, when the first of two devastating Japanese air raids began at 9:58am.

It was the first time a World War II battle reached the Australian mainland.

"The first we saw was a whole heap of bombers coming over, and it wasn't long before dive bombers and fighters, and there was planes going everywhere and explosions everywhere," Mr Ey, now 95, recalled.

"We were near enough to the airfield to hear the bombs coming down."

The men instantly knew what was happening, having expected some sort of attack since Pearl Harbour two months earlier in December 1941, and the fall of Singapore only days before, but not in such a large force.

"We'd been told there would be warnings but it didn't happen, there wasn't any warning, we were absolutely shocked by the force of that bombing," Mr Ey said

Mervyn Lloyd Ey, now 95, survived the bombing of Darwin in 1942-43. ( ABC News: Neda Vanovac )

The attacks involved 188 aircraft launched from four Japanese aircraft carriers located in the Timor Sea, followed by a second wave of 54 land-based bombers, supported by more than a dozen ships, destroyers and submarines.

Private Mervyn Lloyd Ey in Darwin during World War II. ( Supplied: Mervyn Lloyd Ey )

The most commonly cited figure is 243 killed and at least 500 injured or missing, but because many people were buried in hurried graves on beaches or at sea, it is unlikely the true number of dead will ever be known.

Thirty aircraft were destroyed, nine ships in Darwin Harbour and two outside were sunk, and some of the civil and military facilities in Darwin were destroyed.

The Japanese launched 77 attacks on the Top End until November 1943, and along with Australian casualties there were 92 US servicemen who died when the destroyer the USS Peary was sunk under heavy attack in Darwin Harbour.

Sirens will sound once more

Today, at the exact time of the day the raid occurred, sirens will be sounded once again to commemorate the 75th anniversary of the bombing, with military aircraft flying over the city and guns firing in echo of the attack.

Some survivors and veterans of the fight in the Top End have gathered at Darwin's Cenotaph for the service, which is also being attended by the Prime Minister, Opposition Leader and Governor-General.

Basil Stahl, 90, is one of many bombing of Darwin veterans attending the service today. ( ABC News: Xavier La Canna )

"It wasn't a very nice feeling, that was the first air raid I was in, to hear the click, click, click as the bombs were let go," said Cec Jensen, who was an Army mechanic during World War II.

"The most difficult time I think was leaving home and the sweetheart behind, because you didn't know whether you were going to come home in one piece or in a box."

'You had to hope there wasn't a bomb with your name on it'

Bob Andrew was only 21 when he arrived in Darwin 10 days after the bombing to help set up a RAAF defence unit.

"We all had lots of odd jobs; after you'd done a shift, your proper job, you'd find yourself sent off to make camouflage nets, that's how badly supplied we were," he said.

"Supplies and materials from down south were a long way in coming. That partly explains, by the way, helping ourselves, some people thought looting, to try to get materials to build up units that didn't exist."

Being located near the airfield, any overshot Japanese bombs tended to land in their midst, and a few times Mr Andrew was caught out in the open, forced to lay flat on the ground as the bombs exploded around them.

"You just lay low and hope there wasn't a bomb with your name on it. Like most things in war it was the luck of the draw," he said.

"There were two or three fellas killed near where my mate and I decided we'd better lie down ... Your tin hat seemed quite inadequate, but you took what came."

Now 96, Mr Andrew was looking forward to see the change in the city.

"It's a great thrill to see how Darwin's progressed, it's a fine city now, and it was worth all the troubles, I suppose," he said.