Former opposition leader Bill Shorten says he "misread" the level of anxiety caused by Labor's franking credit policy and failed to promise enough tax cuts to people earning under $125,000 a year to win the May election.

Mr Shorten has accepted blame for Labor's surprise election loss as the party prepares to release an election post-mortem in which he is expected to feature heavily.

Former Labor leader Bill Shorten has admitted the franking credit policy cost him votes at the election. Credit:Alex Ellinghausen

The franking credit policy, which would have ended up to $6 billion a year in tax refunds for retired shareholders who had not paid personal income tax, contributed to swings against Labor as high as 15 per in polling booths where people aged over 60 made up more than 15 per cent of the population.

"We misread the mood about franking credits," Mr Shorten said in a press conference on Sunday. "In hindsight there were a lot of people who felt vulnerable."