Party politics and pre-selection issues had taken a toll on Victorian Labor senator Kimberley Kitching before her sudden death, according to former opposition leader Bill Shorten
Senator Kitching died from a suspected heart attack on Thursday aged 52.
A tearful Mr Shorten told ABC Radio that Senator Kitching's death came as a shock and said she had been under great stress.
"Pre-selection is never easy. I'm not a coroner, I can't tell you why this woman at 52 was taken from us, but I have no doubt that the stress of politics and the machinations in the back rooms had its toll," he said.
"There were machinations and arguments going on, now that does happen, that's not unique. But we have a woman who's 52, who's pulled over by the side of the road and died unexpectedly. That's shocking. It's terrible."
Mr Shorten also described how he rushed to the aid of Senator Kitching's husband Andrew Landeryou on Thursday night after being told of her death.
"She was in the next suburb when she passed. So (my wife Chloe and I) immediately drove over, and we sat with a couple of other dear friends of Kimberley and Andrew on the side of the road as we waited for the undertaker's van," he said.
"When someone close to you passes, there's only one thing that you want people to do, it's to bring them back, and you can't."
Victorian Labor colleague Clare O'Neil said Senator Kitching was a "fierce warrior" when it came to internal party politics, always being around the table when key decisions were made.
This was despite much of what goes on in parties being a "bloke's sport".
"I can tell you she was gutsy and fought with everything she had," Ms O'Neil told Sky News.
Tributes have flowed for Senator Kitching from across the political spectrum in the wake of her death.
Opposition Leader Anthony Albanese said her death was an enormous shock.
Mr Albanese said while there were pressures on people in politics, news of Senator Kitching's death was unexpected.
"No one could have anticipated this and people were ringing each other, I think multiple times to get confirmation because it was such a shock," he told ABC TV on Friday.
"Kimberley looked fit, she had got fitter in recent times and there was no history of heart condition and that is why it's such a tremendous shock for the entire Labor family."
Prime Minister Scott Morrison said the senator was a "serious parliamentarian who had a deep interest in Australia's national security".
"She had a passion about Australia's national interest and argued for it," he said.
"She demonstrated that her passion for her country was always greater than any partisan view. She clearly loved her country and it genuinely showed."
Defence Minister Peter Dutton described the late senator as one of the very best in parliament.
"She was a real hawk on national security and I think the parliament will miss her," he said.
Senator Kitching joined the Senate in 2016 and was a member of the ALP for almost three decades.
She was born in Brisbane in 1970 and became a City of Melbourne councillor between 2001 and 2004.
Senator Kitching also worked as a general manager at the Health Workers' Union and as a special counsel at legal firm Cornwall Stodart.
She chaired parliament's Foreign Affairs, Defence and Trade References Committee and was the deputy chair of the Foreign Affairs, Defence and Trade Legislation Committee.
Australian Associated Press
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