THE mystery man of the Siev X affair has broken his silence to defend the Australian Defence Force decision not to step up its surveillance to look for the doomed boat.

Rear Admiral Raydon Gates has chaired an internal defence force review of all classified intelligence received on the boat Siev X which sank en route from Indonesia to Christmas Island in October last year, drowning 353 asylum-seekers.

However, Defence Minister Robert Hill has repeatedly blocked Admiral Gates from testifying on Siev X to the children overboard inquiry. Admiral Gates finally spoke yesterday, saying he believed the intelligence that the defence force received on Siev X was not conclusive enough to act upon.

'No one is denying it was a great human tragedy but let me stress to you, it wasn't an oversight by the navy,' said Admiral Gates at Rockingham, Western Australia, where he was attending the welcoming of HMAS Canberra after five months in the Persian Gulf.

Admiral Gates said intelligence received on asylum-seeker boats was 'far from an exact science'.

'It certainly was not enough from my opinion as an operational commander to start to move assets to that effect.'

Admiral Gates said the ADF received 'a constant flow' of intelligence on all asylum-seeker boats heading for Australian territory.

'Maritime command knew of Siev X as one of many (and) they just had sketchy information.'

The Senate committee has asked the Australian Defence Force to explain why it did not respond to intelligence reports on October 18 and on October 20 that Siev X had set sail.

The intelligence report on the morning of October 20 added that Siev X was dangerously overloaded and the officer conveying this report told Coastwatch she believed Siev X might be at risk of sinking.

That report convinced the ADF's Joint Intelligence Centre that Siev X had set sail.

At this time -- unknown to the ADF -- survivors were still clinging to the wreckage of Siev X, which had capsized and sunk the previous afternoon. A P3 Orion spy plane was passing over the area at the time but did not see any survivors and was unaware that Siev X had sunk.

The Senate inquiry was told this week that this intelligence about Siev X being overloaded and on its way to Christmas Island was not passed to the spy plane crew until the next day -- October 21 -- after the last survivors had been rescued by Indonesian fishing boats.

The Senate committee report later this month is expected to make a key judgment on whether the ADF's decision not to increase surveillance to look for Siev X was justified in light of the information it held at the time. Committee members are likely to criticise the flow of intelligence between government agencies during the time of the Siev X tragedy after testimony revealed that not all information was fully shared.