Mr de Castella, director of the Indigenous Marathon Project (IMP), had travelled to Boston with the project's chief executive Tim Rowe, and Indigenous runner Emma Cameron. Both Mr de Castella and Mr Rowe had finished the race and were in the race hotel when they heard the two explosions near the finish line.

He told ABC radio the situation in Boston was “crazy” as he and Mr Rowe searched for Ms Cameron after the blasts.

“Everything’s pretty chaotic here and pretty crazy as you’d expect,” the Canberran said.

“[It] actually sounded like enormous thunderclaps, and you could feel the vibrations through the room,” he said.

“No one was really sure what was going on, it was pretty chaotic. A lot of the organisers from the Boston Athletic Association who organise this event were pretty shattered.

“It’s a terrible thing to witness, to realise that this sort of thing can take place.”

Ms Cameron, a runner from near Darwin, has been training with the IMP, and competed at the Stromlo Running Festival in Canberra in February. According to an IMP statement, she had made it to the 41 kilometre mark in Boston, just over 1 kilometre from the finish line, when the bombs went off.

She was evacuated from the course, and has since been reunited with Mr Rowe and Mr de Castella.

The trio will return to Australia on Thursday morning.

According to the Boston Athletic Association website, there were 156 Australians registered to take part in the event across all categories. No Australian deaths or injuries have been reported.

Australian paralympian Kurt Fearnley had finished fifth in the race and was also in the hotel when he heard the blasts. He said police locked down the area as authorities scrambled to react.

Concerned friends and family can contact the Department of Foreign Affairs on 1300 555 135.