A boat carrying 45 Tamil asylum seekers was found drifting off Christmas Island yesterday, ABC reported.

An Australian defence ship was sent to rescue a boat near Christmas Island after it sent out distress calls yesterday afternoon.

The Refugee Action Coalition says there are 45 Tamil asylum seekers on board the boat, which has been drifting for four days without food and water.

Spokesman Ian Rintroul says he expects the asylum seekers will be taken to Christmas Island detention centre.

"It will add to the congestion on Christmas Island without doubt," he said.

"It's going to be an enormous relief for these people to actually have assistance but of course it's an extra 45 people added to an already crowded facility.

"And no doubt there would be even better facilities if the people had been brought straight to the mainland.

"They'd left from Sri Lanka on the 20th of January I was told, but have now run out of fuel and the boat's drifting.

"They'd been without food or water for the last four days and people are quite sick. There are a couple of cases of chicken pox and diarrhoea."

The Australian Maritime Safety Authority expects the defence ship to arrive with assistance within the hour and assess the situation.