A last-minute secret side probe is responsible for the NSW ICAC's latest delay in Gladys Berejiklian probe, documents reveal.

Dominic Perrottet’s ties to one of the decade’s biggest political scandals likely remain hidden from voters until after March election.

NSW Premier Dominic Perrottet is refusing to stand by the legality of a $5.5 million grant he made following an “intervention” by Gladys Berejiklian and heavy lobbying by Daryl Maguire.

The $5.5 million grant to fund a 1000-person convention centre at a gun club in Wagga Wagga was the only one - of over 90 grants - in the NSW Murray-Riverina region that went to a non-government entity.