Revealed: Helen Kelly blamed fatal bomb attack on Sir Robert Muldoon

A new film about the late union leader Helen Kelly reveals she blamed a fatal bomb attack on anti-union hysteria whipped up by former Prime Minister Sir Robert Muldoon.

The movie is based on a series of interviews with the union leader in the year before she died - including one about the 1984 Trades Hall bombing.

There is a scene on the bomb attack, based on interview footage released to Newshub, in which Helen Kelly clearly blames it on the environment created by Sir Robert, known as 'Muldoonism'.

"Just this absolute sort of anti-Communist, anti-socialist, anti-reds under the bed hysteria which was really designed to shut down trade unions and discredit them," she said.

Ms Kelly said the frenzy led the suitcase bombing left in the building, its explosion killing caretaker Ernie Abbott.

"It was run by Muldoon - and it was vicious, and people were being forced to be sort of scared of trade unions and to see them as a threat," she said.

"And it got worse and worse, and it was this sort of war of words at that stage - and then suddenly, someone put a bomb in the Trades Hall."

It was a rare act of terrorism in New Zealand - but has never been solved.

"No breakthrough, no breakthrough, theory after theory, story after story," she said.

The movie, to be released in September, is based on documentary-maker Tony Sutorious following Ms Kelly in the year before she died from lung cancer.

In it, she recalls how she knew Mr Abbott well - her father Pat Kelly worked in the hall with him.

"He was just a lovely guy who lived in the hall, did his job, was a union person as well, had a little dog which everybody loved that got injured in the bombing, Patch."

In the movie interview, Ms Kelly is drawing attention to another era - and another injustice in New Zealand.

Because 34 years on - the bomber of the Trades Hall, a killer, a murderer - has never been found.