The extension of time for the enhanced first home grants puts more borrowers at risk of default, as the spectre of double-digit unemployment looms, an academic says.

University of Western Sydney Associate Professor of Economics and Finance, Steve Keen, said the federal government's decision to extend the period for the enhanced grants would benefit sellers.

'It is really asking first home buyers to take out leveraged positions to boost the economy at a time when it is very likely the economy is going to go very cactus and so are house prices,' Professor Keen said.

'I am calling it the first home vendor's grant.'

The Age