LABOR will be clobbered by voters at the Penrith byelection, with secret ALP polling obtained by The Sun-Herald indicating Premier Kristina Keneally will preside over the greatest-ever swing against a NSW government.

In an ominous sign for the scandal-plagued state government before an election in March, internal party polling predicts a massive two-party preferred swing of 27 percentage points against the government in Penrith on June 19.

The slaughter is set to be worse than the previous greatest swing in NSW, when John Watkins's former seat of Ryde returned to the Liberals last year with a 23-point two-party-preferred swing.

On a two-party preferred basis, Labor is on track to claim just 32 per cent of the vote, with 68 per cent for the Liberals representing a stunning turnaround from the 2007 election when disgraced former MP Karyn Paluzzano won for Labor with 59 per cent of the vote.

The results of the polling have left ALP strategists reeling and senior government figures clutching for reasons why things cannot get any worse for the ageing Labor administration 10 months out from an election.