The Victorian stalwart died on Thursday evening of a suspected heart attack, aged 52, on the eve of the federal election campaign.

Tributes poured in from colleagues from all sides of the political spectrum, praising her "warm" personality and unrelenting approach to national security and foreign affairs issues.

In her more than five years as senator, she made a name for herself as the Labor figure that couldn't easily be boxed.

Former Labor opposition leader, and close personal friend, Bill Shorten described her as a politician with a "spine", which meant she wasn't always liked within her own party.

On Friday morning, Mr Shorten told radio stations people in the Labor Party "would give her a hard time" because she stood up for what she believed in and "wouldn't cut the cloth of her views to suit the fashion of the day".

She had been elected senator in 2016 - a "captain's pick" by Mr Shorten - but factional issues within the Victorian branch were threatening to unseat her.

Mr Shorten added she was under great stress and recent party room machinations "had its toll" on her.

But outside of Labor, she was revered for her ability to work across party lines.

Senator Kitching was able to find the common ground with senators her colleagues would consider foes.

She struck an unlikely friendship with controversial One Nation senator Pauline Hanson, who described having Senator Kitching join her in the office for afternoon teas during sitting weeks.

The two had "a hoot" and bonded while they were on a parliamentary trip to Afghanistan in 2018.

"There's only a couple in that parliament that I hold in such high regard," the One Nation senator told The Canberra Times on Friday.

"One on [Liberals], one on Labor. She was the one on the Labor side."

Conservative Tasmanian Liberal senator Eric Abetz said she "bubble-wrapped" their personal friendship from their political disagreements.

Senator Abetz said he would tell her she had an "assassin's smile" - a pleasant smirk that should not be underestimated.

The two were often seen having lunch together at parliamentary meeting places, such as Aussies Cafe.

"Not only have I lost a colleague, but I consider I've also lost a friend," he said.

"She was a cut above the rest."

Beyond politicians, the senator was also in regular touch with key embassies.

Both the US and Japanese embassies issued statements on Friday, with the latter's ambassador Shingo Yamagami adding he was "deeply saddened" and that "he very much appreciated her friendship".

The Victorian senator played an instrumental role in an inter-party push to adopt Magnitsky-style laws that would allow Australia to impose sanctions on individual and entities for human rights violations.

The legislation passed late last year and has been used recently to target Russian banks, military officials and President Vladimir Putin's inner circle in response to Moscow's unprovoked invasion of Ukraine.

British financier Bill Browder, who led the global charge to bring in the sanctions, described the senator's sudden death as a "terrible tragedy" for Australia and the world.

The bill was named after Mr Browder's tax adviser Sergei Magnitsky, who was killed by the Kremlin in 2007 after revealing widescale corruption and misconduct within Mr Putin's regime.

Mr Shorten said Senator Kitching's forethought to campaign for it years earlier, before the recent invasion, showed the type of "conviction politician" she was.

"She raised it when it wasn't sexy, when people didn't know who Putin was or where Kyiv was," he told radio stations.

Senator Abetz, who worked in tandem with her across committees, said her level-headed approach made for an easy working environment.

"She was in the vanguard of that movement, and, if I might say, her sort of sensible, considered approach helped us get it through the Parliament," he said.

"The fact that we've now got it through is fantastic in helping us deal with Russian oligarchies."

Greens senator Janet Rice said the passing of the sanctions regime was one of those rare moments in the senate which transcended the chamber's vast political views.

Like the earlier push for marriage equality, which both senators were also a part of, Senator Rice said it was a moment where politicians dropped the usual charade to hug and congratulate each other.

"It's part of her legacy to have been a really strong, powerful force in getting the Magnitsky legislation across the line," she said.

National security expert Professor John Blaxland never met the senator but added her contributions to the space were "significant" and "important".

But it wasn't always without controversy.

Her hawkish approach to national security landed her in hot water.

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She was controversially a member of The Wolverines, an informal parliamentary group focused on the threat China poses to Australia's sovereignty.

In a recent senate estimates appearance, she used parliamentary privilege to name Chau Chak Wing as the "puppeteer" in a Chinese foreign interference plot foiled by ASIO.

She also played a key role in revealing the Cartier watch gifting scandal that brought down Australia Post's former head, Christine Holgate.

Under questioning by Senator Kitching, it was revealed Ms Holgate had gifted four Cartier watches to senior executives for their work on a major deal.

It caused a political storm, which led to Ms Holgate being stood aside, later being sacked, while an independent investigation was undertaken.

The senator later said she didn't regret her role in revealing the saga.

Ms Holgate, now the chief executive of competitor Toll Global Express, offered her "deepest sympathies" to Senator Kitching's family over the "very tragic loss".

But independent senator Rex Patrick, who worked alongside her in defence-related committees, said she was a true believer in what she stood for and wasn't looking for easy political wins.

He added she was a prolific senator, who placed more than 2000 questions on notice with government officials over the nearly six-year term - far exceeding most senators.

"She was always much more sophisticated than what I am, partly because she spoke French," he joked.

"Where our issues aligned, she was just great to work with and you respected where they didn't align.

"I think the Labor Party will be worse off with her not being there."