The Abbott government has spent $2.5m on an unknown number of disposable lifeboats to send asylum seekers back to Indonesia.

Border protection authorities purchased the lifeboats by piggybacking on existing government contract tenders, the customs and border protection chief Mike Pezzullo told a Senate estimates hearing in Canberra on Tuesday.

The money was allocated in the mid-year budget review, released in December.

Pezzullo would not say how many lifeboats had been purchased.