Australia’s biggest steel maker is suing the Australian Workers Union over an unauthorised walk-off.

Bluescope Steel is seeking millions of dollars in penalties or compensation after the unsanctioned 24-hour strike by workers at Spring Hill in May.

The AWU's Port Kembla South Coast and Southern Highlands Branch has received written notice of the Federal Court action, and said it estimated that the claim would be in the order of $2.4 million.

Branch secretary Wayne Phillips has told the ABC that workers had demanded a meeting about management practices before they took the industrial action.

“Changing their rosters, getting them to work everywhere, changing their crib allowances, all sorts of things,” Mr Phillips said.

“They just continually attack. There's over 24 casuals working on site yet at the same time they are reducing numbers.

“They just had enough so we had a meeting of the members. The members overwhelmingly voted to go on strike for 24 hours.”

Mr Phillips – who is personally named along with the AWU Port Kembla branch and another organiser in the court documents - acknowledged the illegality of the industrial action under the Fair Work Act.

“That was certainly discussed with our members, but sometimes people get themselves that cranky and that fed up,” he said.

“This was the plant that the company acknowledged last year, in all the hassle we had last year, was making good money for them, keeping them afloat.

“They have done nothing but tap the workers of their conditions ever since.”

The AWU says Bluescope has changed since a well-received deal was struck last year, which saw more than 400 jobs cut to save the struggling Port Kembla steel works.

Mr Phillips says that positive relationship between the union and management has now disappeared.

The AWU branch is expected to fold if Bluescope succeeds in the claim, but the union has vowed to defend it in court regardless.