WATERLOO — A local union broke the law when it paid 23 members $100 each to stand behind Liberal leader Justin Trudeau when he campaigned here last Sept. 15.

That's a no-no because it's like the union making a $2,300 donation to the Liberals, which is unlawful. The Canada Elections Act says only citizens and permanent residents can make political donations.

"We didn't know that we were breaking the law," said Russ Jessop, business manager for United Association Local 527, a union representing plumbers and pipefitters. "We feel bad. But now we know. And we'll make sure we let everybody else know, now that we know the rules."

The union has voluntarily admitted its offence and has agreed it will not happen again. Canada's elections commissioner has accepted the pledge. It's called a compliance agreement and it concludes an investigation into the offence, with no conviction recorded.

Trudeau came to Local 527 hall on Frobisher Drive and pledged to invest $750 million to help train skilled workers. According to the compliance agreement, the Liberal Party asked the union to have workers there. The union responded by writing 23 cheques to its members for attending as props.

Had members volunteered without payment it would have been lawful. Liberal supporters and candidates also attended.

The Liberal Party, which won the election, did not know the union was paying members to attend, the compliance agreement says. The party has since paid $2,300 to the government. The agreement says the union co-operated fully in the investigation.

Someone complained soon after the event to the commissioner of Canada Elections. A spokesperson for the commissioner said the complainant can't be identified.