The families of two men killed during World War II gathered at dawn at a beach in Broome for a memorial service they have waited 70 years for.

Flight Sergeants Ronald Smith and Ronald Kerrigan were killed in September 1944 when their RAAF Beaufighter crashed just after take-off in the waters off Broome.

But for many years the plane wreck could not be found, and their bodies were never recovered.

Two years ago two amateur historians decided to solve the mystery once and for all, beginning a painstaking search using a small boat and donated sonar equipment.

In June, they were able to phone the men's families with the news they had been waiting for — the wreck had been found.

Now a memorial plaque has been erected high on the grass overlooking Cable Beach, less than two kilometres from the men's final resting place.

More than a dozen relatives of sergeants Kerrigan and Smith travelled from across the country for a dawn service to see the plaque unveiled on the 70th anniversary of their deaths.

Val Bullied was 15 years old when she got the news her brother Ronald Kerrigan had been killed.

She said it was an emotional morning.

"It's just overwhelming and I can't show how appreciative I am. It's so amazing," she said.

"It's nice that they're recognised. You feel now that people know that they're there."

The memorial at Cable Beach is about two kilometres from the men's final resting place. ( ABC: Richard Dinnen )

Her brother had been studying accountancy when he enlisted for the RAAF.

He was 20 years old when he died.

"My brother has never left me, and I remember him just like he is in these photographs; see him walking towards me on the street. It's hard to explain," Ms Bullied said.

"We had no money but we had a good life. We used to have fun, made couple of jam cans with a string to make telephones in the backyard. All the things kids did in those days when they had no money but we had a good life.

"He was something special, and I still love him."

Ms Bullied said not knowing where her brother lay gnawed at the family.

The two men who died were good friends, and the men's mothers kept in touch via handwritten letters in the years after.

A final closing for a mission: RAAF

RAAF Wing Commander Joanna Elkington attended the memorial service.

She said it was a significant moment for the families and the Air Force.

"The significance of this kind of ceremony is that the families have closure on what's actually happened to their family members, and for us it's a chance to acknowledge the sacrifices they've made, and remember them," Wing Commander Elkington said.

"I'm really glad to be able to be here as the commanding officer of 31 Squadron and actually share with the families a final closing for that mission."

It was a sad but satisfying moment for the two men who spent long hours at sea to locate the plane wreck.

Broome historian Dion Marinis made contact with Darwin-based Jim Miles when he realised they were both interested in solving the mystery.

"It's been a long road for Jim and I carrying out the searching, but for it to come to this is like the icing on the cake," he said.

"It's very important for the families so it's great to be able to assist them in having some closure."

They are now turning their attention to several new searches.

Mr Miles said there were dozens of undiscovered World War II plane wrecks dotting the Australian coast.

"There are significant numbers of wrecks off the WA coast, in particular, that remain lost, and we're keen to find more," he said.

"There's a Liberator bomber that crashed off Broome, and we've already commenced a lot of work trying to locate it.

"It's a work in progress so watch this space."