Former prime ministers Kevin Rudd and Julia Gillard will appear before the Abbott government's royal commission on the abandoned Home Insulation Program if requested but will not answer questions relating to cabinet deliberations.

The distinction means the $12 million commission has little prospect of learning how the scheme was conceived within the top levels of the Rudd cabinet, and of the extent of diligence associated with its planning.

Lawyers contacted by Fairfax Media said it remained unclear how the commission, which enjoys extraordinary powers to compel witnesses, would respond to a blanket refusal by former ministers to discuss matters so central to its terms of reference.