Lawyers are warning about a big increase in children "bullying" their parents into signing mortgage contracts or, in the case of ageing parents, simply helping themselves to their cash and investment accounts.

Soaring property values, lender demand for bigger deposits and tougher terms are contributing to a 25 per cent increase in contributions by the so-called Bank of Mum and Dad (parents lending to their children for home purchases) to about $20 billion in the past 12 months.