B.C.’s biggest public sector union is going to the province’s Supreme Court to fight heavy financial penalties levied for illegal election-related advertising.

Elections BC last month fined the B.C. Government Employees Union more than $3 million for running advertisements during two provincial byelections in April.

The agency calculated that the BCGEU had exceeded its $3,000 spending limit by nearly $160,000 per electoral district, and the fine is calculated as a compound of those amounts.

According to a petition filed with the court, the union claims that the ads were planned as part of a campaign leading up to the expiry of collective agreements, according to BCGEU spokesman Chris Bradshaw.

Bradshaw said the union had no way of knowing the government would drop writs for byelections at the same time.

“Elections BC erred in its calculation of those fines. We also believe that their decision to characterize the advertising as election advertising was wrong. And those are two of the core elements of our case,” said Bradshaw.

"It's a substantial amount of money for any organization, there's no question."

Also as part of the Elections BC penalties, the BCGEU would be forbidden from running election ads during the next provincial election in May 2013.

The union is asking the court to overturn the decision. Elections BC has yet to file a response to the civil claim.

An estimated 300 B.C. Government and Service Employees' Union members walked off the job on Monday as part of the union's efforts to pressure the government to resume stalled contract negotiations.

With files from the CBC's Jason Proctor