Luxury superyacht Equanimity is said to be the 54th biggest yacht in the world.

At 90 metres, it has a swimming pool on deck, a helipad, movie theatre, spa and sauna — and cabins for 28 crew and 18 guests.

Its interior is apparently lined with marble and gold leaf, according to website Yacht Charter Fleet.

So when the FBI tipped off Indonesia the massive yacht was in waters somewhere near Bali — and they suspected it was bought with stolen funds — it was not too hard a task to find it.

Indonesian police traced the yacht to the island of Lombok last weekend, then seized it as it sailed into Bali.

It has been impounded at Benoa port in Bali's south, while police investigate its ownership and the crew.

Agung Setya, director of economic and special crimes at Indonesia's criminal investigation bureau, says the 34 crew — including two Australians — have been kept on board while investigations proceed.

"We know that only the crew were on board," he said.

"We are checking the ship's captain to know more about what they were doing in Indonesia.

"We're still investigating."

Bali police on Thursday questioned the South African captain and multinational crew on behalf of US authorities.

Police are checking the crew's immigration papers to see if there is evidence of any crime.

Superyacht Equanimity has been impounded as police investigate its ownership and the crew. ( Reuters: Antara Foto/Wira Suryantala )

Yacht 'bought with embezzled state-owned funds'

But it is the money used to buy the yacht that most interests the FBI.

US authorities believe it was bought with some of the $US4.5 billion that was allegedly siphoned from Malaysian Government-owned sovereign wealth fund 1MDB by high-level officials at the fund or their associates — including Malaysian financier Low Taek Jho, better known as Jho Low.

Najib Razak set up 1MDB and later chaired its advisory board. ( Reuters: Olivia Harris, file )

It is the same government fund at the centre of a corruption scandal that has tainted Malaysia's Prime Minister Najib Razak — who set up 1MDB in 2009 and later chaired its advisory board.

Mr Razak has consistently denied wrongdoing, despite investigations into the source of millions of dollars that have poured into his personal bank account in the years since.

One of Mr Najib's close associates, Mr Low is accused in US court documents of misappropriating billions of dollars from 1MDB to buy the yacht and other assets including a private jet, a US hotel and millions of dollars in jewellery.

The US Department of Justice (DOJ) alleges $US1.7 billion of the money was laundered through US financial institutions.

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Police checking for evidence of crime

Indonesian police received a letter from the FBI on February 21, requesting help to enforce a court order to trace the yacht.

They say information from the FBI showed the yacht's Automated Identification System had been switched off several times in waters around the Philippines and Singapore, making it harder to track.

Over the past 180 days, the Equanimity is believed to have sailed from Malaysia and Singapore to Bali, then to Papua and back.

But there were several points where its movements were unclear.

"We're co-ordinating with the US to get more information," police investigator Agung Setya said.

"We know it's the proceeds of money laundering that have bought not only the boat, but we're aware there's more."

Miranda Kerr, Leonardo DiCaprio 'given assets bought with 1MDB funds'

Other assets bought with the stolen 1MDB funds have since been recovered, including millions of dollars in jewellery Mr Low allegedly gave to Australian model Miranda Kerr when she went out on the yacht more than a year ago. She later handed the jewellery over to US authorities.

Mr Low allegedly gave Miranda Kerr millions of dollars in jewellery when she went out on the yacht. ( AAP: Tracey Nearmy )

Actor Leonardo DiCaprio also handed back an Oscar once owned by Marlon Brando.

Indonesian police have searched the yacht and say Mr Low was not on board.

The Malaysian businessman has previously hit out at the allegations against him, saying last year the US was yet to prove he was guilty of any impropriety.

On Thursday, a spokesman told Malaysia's New Straits Times it was "disappointing that, rather than reflecting on the deeply flawed and politically motivated allegations, the DOJ is continuing with its pattern of global overreach — all based on entirely unsupported claims of wrongdoing".

Mr Low's whereabouts are unknown, although the head of Indonesia's National Police special crimes investigation unit says Indonesian authorities are not looking for him.

Authorities in Bali were yet to reveal how long the yacht will be impounded, what they will do with it, or when the crew will be released.