A mysterious slush fund linked to the shop assistants' union - a dominant player in the Labor Party - was used to back a ticket aligned with whistleblower Kathy Jackson and federal Labor frontbencher David Feeney in the elections of the disgraced Health Services Union.

A Fairfax Media investigation has revealed a slush fund connected to the Shop Distributive and Allied Employees Association bankrolled the Jackson/Feeney-linked ticket in an election in 2012.

Kathy Jackson.

The slush fund contributed at least $30,000 to the ticket led by Marco Bolano, a former union secretary and ally of Ms Jackson.

The royal commission into union corruption will examine evidence that the shop assistants' union - or slush funds associated with it - helped finance a successful Bolano campaign in 2009. Fairfax Media believes the sum was in the tens of thousands.