Police have released Melbourne Cup-winning horse trainer Darren Weir and two other men without charge as they investigate allegations relating to the corruption of betting results and animal cruelty.

Key points: Darren Weir is widely regarded as Australia's leading horse trainer

Darren Weir is widely regarded as Australia's leading horse trainer Police arrested him and raided his properties as part of an investigation into racing integrity

Police arrested him and raided his properties as part of an investigation into racing integrity They say they also found electric-shock devices, which may have been used on horses

Victoria Police raided properties owned by Weir in Warrnambool and near Ballarat, and arrested Weir, 48, and two other men aged 26 and 38.

Assistant Commissioner Neil Paterson said the raids formed part of an ongoing investigation into allegations of conduct that corrupts or would corrupt a betting outcome.

The investigation has been underway since August.

Officers searched Weir's high-tech training facility at Miners Rest and his beach stable in Warrnambool, as well as a property at Yangery just outside Warrnambool.

Darren Weir embraces Michelle Payne after winning the Melbourne Cup in 2015. ( AAP: David Crosling )

Assistant Commissioner Paterson said officers found what was believed to be an unregistered gun and four devices known as "jiggers", which can deliver electric shocks.

"[In relation to] the conducted energy devices, or the jiggers, clearly the allegation is that they may be used against a horse with the aim of improving their performance on a particular race day," Assistant Commissioner Paterson said.

"I have heard that there may be footage in the public arena … of jiggers being used, and if that's the case my call would be to anyone in the public arena, if they have footage, that they come forward and provide that to Victoria Police."

Cocaine was also found at a property searched by police, he said.

Weir is widely regarded as Australia's leading trainer and trained the Melbourne Cup-winning horse Prince of Penzance, which jockey Michelle Payne rode to victory in 2015.

Police searched a Darren Weir Racing truck at the horse trainer's stables near Ballarat. ( ABC News: Sian Johnson )

Assistant Commissioner Paterson said he could not go into further detail about the alleged actions of the three men as investigators were still gathering enough evidence to lay charges relating to:

obtaining financial advantage by deception

obtaining financial advantage by deception engaging in conduct that corrupts or would corrupt a betting outcome

engaging in conduct that corrupts or would corrupt a betting outcome use of corrupt conduct information for betting purposes

use of corrupt conduct information for betting purposes attempting to commit an indictable offence

attempting to commit an indictable offence offences related to animal cruelty

None of those charges have yet been laid and all three men have been released from custody pending inquiries.

"I can't go into the specifics that give rise to the circumstances of each one of those charges because of the complexity of this investigation and because it is ongoing," he said.

Darren Weir's facility at Warrnambool was also raided by police. ( ABC News: Daniel Miles )

Joint investigation between police and Racing Victoria

Racing Victoria CEO Giles Thompson said the racing body approached police with the allegations last year, which prompted the investigation by Victoria Police's Sporting Integrity Intelligence Unit.

He said the decision on whether or not Weir's horses would be allowed to run while he was under police investigation would rest with stewards.

"At the moment, there is no information or suggestion to the stewards that there should be any reason that the horses shouldn't be running or anyone should be stood down at this point in time," he said.

"No charges have been laid and thus those involved are entitled to the presumption of innocence there."

Who is Darren Weir? Born in the town of Berriwillock in Victoria's north-west

Born in the town of Berriwillock in Victoria's north-west Left school at 15 and began work in the horseracing industry

Left school at 15 and began work in the horseracing industry Took out his own trainer's licence aged 25, when based at Stawell

Took out his own trainer's licence aged 25, when based at Stawell Expanded operations to Ballarat and Warrnambool as his success grew

Expanded operations to Ballarat and Warrnambool as his success grew Won 348 races in the 2015-16 season, setting a new Commonwealth training record

Won 348 races in the 2015-16 season, setting a new Commonwealth training record Trained the 2015 Melbourne Cup winner Prince of Penzance

He would not speculate on whether or not the allegations, if substantiated, would result in retrospective changes to past race results.

"As you'll appreciate, the activities of today, the dust is still settling on those and investigations are still ongoing and still fairly intense and at this point in time we don't have anything to add."

Mr Thompson said the investigation demonstrated Racing Victoria's commitment to the integrity of the sport.

"What we need to do is to continue to show the public that we're committed to the integrity and the welfare of the horses in racing, and that's what this is showing very clearly."

Darren Weir expanded his stable empire recently with the acquisition of Trevenson Park in Maldon. ( ABC Central Victoria: Larissa Romensky, file photo )

'Extremely disappointing': Racing Minister

Victoria's Racing Minister, Martin Pakula, said he was aware of the police investigation and did not want to say anything to prejudice its outcome.

"I would simply say as a racing fan and as the Racing Minister, integrity in our sport is incredibly important and any allegation of a breach of the rules of racing is extremely disappointing," he said.

"It is important to note that this is another demonstration that our racing integrity unit within Racing Victoria will pursue any matter without fear or favour, and that is clearly what they have done."

Racing journalist Matt Stewart, from Melbourne radio station RSN, said that there had been “rumblings” in the industry that Weir's stable was being closely monitored.

Even so, he was surprised by the "gravity" of the alleged offences being investigated.

"Darren Weir was the fairy tale trainer, is the fairy tale trainer," he said.

"He is numerically the biggest trainer. He has about 400 horses in work.

"He has a lot of concerns on his plate right now, as does the racing industry."

Staff continued to come and go from the Miners Rest property throughout the morning, with regular work continuing despite the presence of police.

Weir also owns Trevenson Park, a property south-west of Bendigo, which includes a large dam and a purpose-built track made of sand.