An Australian Customs patrol boat had to rescue about 40 asylum-seekers at the weekend after accidentally ripping the bow of the boat away and causing it to start sinking.

Multiple sources have told Fairfax Media that the patrol boat was trying to tow the fishing vessel when it damaged the front of the vessel, causing it to take on water.

The fishing boat's engines had stopped about 20 kilometres off Christmas Island but it is unclear whether Customs was at the time of the incident trying to tow the boat back towards Indonesia or to Christmas Island.