The British investigative journalist is now back in the country of her birth after being blacklisted for years, and being treated as a celebrity in a sign of the whirlwind changes since historic May 9 elections.
No one is more stunned than Rewcastle, who said she expects to see further startling revelations of corruption and misrule emerge as a reformist administration cleans house.
"There is so much that’s going to come tumbling out now," she said during an interview in Kuala Lumpur.
"Everyone is gob-smacked as they see these things happening. There are going to be more amazing scenes to come."
Rewcastle, now 58, has been a thorn in the side of Malaysia's ruling elite for years, working from abroad to expose larceny and misrule centring mostly on the rainforested state of Sarawak where she was born and spent her early years.
But her biggest bombshell may have been the 2015 revelation by her website Sarawak Report that nearly $700 million was funnelled into the bank account of ex-premier Najib Razak.
That helped super-charge allegations that Najib and his entourage plundered billions from sovereign wealth fund 1MDB, in a scandal that led to his electoral defeat, ending six decades under an increasingly corrupt government.
He is now under investigation and expected to be charged.
Rewcastle's work over the years triggered Malaysian arrest warrants, lawsuits, threats, and a sustained campaign of online vilification that she suspects was orchestrated by Najib's government using western PR firms.
The sister-in-law of former British prime minister Gordon Brown, Rewcastle was still recently being approached by shadowy characters offering pay-offs if she'd publish juicy "revelations" for them -- ham-fisted attempts to entrap and discredit her, she says.
"Millions have gone into trying to destroy my reputation, which could have been spent on something useful," she said. "But all they did was help make me famous, the stupid idiots."
Never welcome, and officially barred from Malaysia in 2015, Rewcastle has gone almost overnight from persona non grata to welcome guest.
She met AFP following an interview with a state-aligned newspaper that formerly maligned her but gave her glowing front-page treatment on Monday.
She was halted repeatedly by ordinary Malaysians who recognised her distinctive ginger locks, stopping to thank her and snap selfies.
Many more have praised Rewcastle on social media after learning of her arrival. "It's extremely gratifying," she said.
Few foreigners were as feared by Malaysia's government.
Born in Sarawak when it was a British crown colony, she spent several years there, often following her mother -- a midwife for indigenous people -- on jungle jaunts to remote clinics.
She later worked for the BBC and others in London in investigative journalism before devoting herself to publicising Sarawak corruption, deforestation, and eviction of native peoples from traditional lands.
"I did this partly because I was mad, and partly because I thought there was a slim chance something could be done," she said of the state which environmentalists believe has lost nearly all of its original rainforest.
In 2010, she started Sarawak Report and short-wave broadcaster Radio Free Sarawak -- operated in secret from London, and later Bali, Brunei and Sarawak itself.
Rewcastle drew on a network of contacts in Malaysia to repeatedly expose the plundering of Sarawak. Najib's regime eventually blocked the website -- a move the new government has reversed -- and radio signals were jammed.
With Malaysia on a reform path, Rewcastle expects to wind down her anti-graft work, which she said was a money-losing project reliant on financial backers she won't name.
But she pledged to "do my darnedest" to continuing advocating for Sarawak.
That includes pushing for investigations into its former chief minister, Abdul Taib Mahmud.
The retired 82-year-old, who was loosely aligned with Najib's regime, is accused by indigenous activists of ruling Sarawak like a family fiefdom for 33 years, plundering its timber and building ecologically harmful dams.
Sarawak Report, along with the Bruno Manser Fund, a Swiss NGO, has documented huge investments around the world by Taib's circle.
"Taib needs to be taken by the ankles and shook, so the money falls out," Rewcastle said.
"There's still a lot to be done. But we're in a terrific position now to really campaign for what this was originally about."