Labor MPs say they are "cautiously watching" as an investigation begins into a harassment allegation made against Opposition Leader Luke Foley, just five months out from the state election.

But under pressure in question time on Wednesday, Mr Foley threatened government ministers that he would return fire if they pursued the issue, which was first raised in State Parliament last week.

Luke Foley was forced to fight back during question time on Wednesday. Credit:Louise Kennerley

The ABC has launched an investigation into the claim that Mr Foley allegedly got drunk and harassed a female ABC journalist at a function in Sydney's CBD in 2016.

Mr Foley has denied the claim and the journalist involved does not want to comment.