Australian authorities waited more than 24 hours after they concluded a boat carrying nearly 400 asylum-seekers was overdue before they sent a patrol plane over the area.

However, they did not know the boat, known as Siev X, had already sunk three days earlier with the loss of 353 lives.

New evidence to be presented to a Senate committee shows the Australian Defence Force chose not to increase daily surveillance of the region in the 24 or so hours between the time it concluded Siev X was overdue and the time it learned the boat had sunk.

By this stage, the ADF had received specific intelligence that the boat had set sail and was dangerously overcrowded.

Despite this, defence surveillance schedules obtained by The Australian reveal a gap of 25 hours in P3 plane aerial surveillance of the waters near Indonesia -- from 11.25am on October 22 to 12.25pm on October 23.

The head of Coastwatch, Rear-Admiral Mark Bonser, told the committee last month officials finally became convinced Siev X was overdue at 10.03am on October 22.

At that moment, a P3 plane was still flying over the area but Coastwatch did not tell the ADF of the boat's overdue status until 14.05pm, after the P3 patrol had returned to its base.

The next P3 patrol over the area was not until lunchtime the next day, October 23 -- 25 hours after the previous flight had left the surveillance zone.

On that same morning -- shortly before the P3 took off -- Rear-Admiral Bonser said the Australian authorities learned the Siev X had sunk.

Rear Admiral Bonser told the committee that just because the Siev X was overdue did not necessarily mean it was in trouble.

An overdue boat "was not unusual and might be due to a range of factors including diversions", he told the committee.

Coastwatch received intelligence on October 18, and again on October 20, saying that the Siev X had departed Indonesia for Christmas Island, but it was not until two days later that new intelligence convinced it that the Siev X had actually set sail.

"On Monday, 22 October, Australian Federal Police provided further advice to Coastwatch that corroborated the previous advice about the departure of the vessel and that, by now, the vessel should have arrived in Australian waters," Rear Admiral Bonser told the committee.

Upon determining that Siev X was overdue, Coastwatch told both the ADF and the Australian Search and Rescue. Neither took extra action.

The ADF did not change its surveillance patterns, while the rescue service concluded that because no one had received a distress call, or had knowledge of a vessel being in difficulty, no search-and-rescue activity was required.

Four days earlier, Indonesian people-smugglers had crammed nearly 400 asylum-seekers on to the barely seaworthy Siev X -- some of them forcibly -- and sent them on their way.

But the boat sank on the afternoon of October 19, leaving only 44 survivors clinging to wreckage before they were rescued on October 20.