Two boatloads of asylum seekers are believed to be aboard Australian vessels off Christmas Island but the government has refused to confirm their existence or comment on whether they will be allowed to set foot in Australia.

By Sunday evening it had been more than 24 hours since any civilian communication with a former fishing trawler carrying 153 Tamil asylum seekers, including 37 children, that left the southern Indian city of Pondicherry 16 days ago.

No comment on boat arrivals: Immigration Minister Scott Morrison. Credit:Alex Ellinghausen

Refugee advocates believe they have been intercepted by the navy and transferred to the customs vessel Ocean Protector, which is equipped to hold 250 people.

A second boat that left Indonesia is also believed to have been stopped off Christmas Island by the customs ship Triton.