The human rights group has released a report claiming Australian officials paid people smugglers to turn back boats and threatened asylum seekers.

Immigration Minister Peter Dutton has rejected the findings, calling Amnesty a disgrace.

Amnesty International says its findings are "damning" and calls Australia's maritime border-control policies a "lawless venture".

The human-rights group's report is entitled 'By Hook or By Crook', a phrase coined by former prime minister Tony Abbott to describe Operation Sovereign Borders.

It refers to two key dates in its findings.

In May, Amnesty claims more than $40,000 was handed over to crew members intercepted on a boat on its way to New Zealand.

It says that money was paid to get the crew to return to Indonesia instead.

The report's author, Anna Shea, says her research was comprehensive.

"I spoke with everyone who had been on board that boat, the boat that was turned back in May, and they said that the crew had received money. The crew told me that they had received money. The Indonesian police told me that they found money on the crew, and they showed me the money. So, this is all very serious and compelling evidence. The asylum seekers told me that they were held on a Border Force ship in very crowded, hot, airless cells, they were locked inside, they were denied access to medical treatment."

The Indonesian government has previously stated it believes Australia paid the ship's crew during that operation.

Meanwhile, in July, Amnesty alleges Australian officials intercepted a boat and separated the two Indonesian crew and 15 passengers on a Border Force ship for a week.

Ms Shea says there are questions whether money was also exchanged during that encounter.

"We don't have the same level of proof as we did for the May incident, but asylum seekers told me that, after they were transferred onto a new boat by Australian officials after being detained, they said the crew members had bags that they didn't have before and that Australian officials seemed very, very worried about anyone touching these bags. The asylum seekers told me, 'We're worried that there was payment in this case.' But we can't say for sure. And that's just another reason for that point to the need for an investigation to get to the bottom of what's happening on the high seas."

Immigration Minister Peter Dutton has denied the allegations and says people are lawfully intercepted at sea under safe conditions.

Speaking on 2GB radio, he accused Amnesty of casting a slur on border officials.

"They don't like Operation Sovereign Borders. They try and attack the Border Force staff and the naval staff, and I think it's a disgrace. I think, in the end, you can take the word of the people smugglers or you can take the word of our staff at Australian Border Force, and people will make their own judgments."

Foreign Minister Julie Bishop has also rejected the report.

It comes as a Senate inquiry continues to investigate the May operation, but Amnesty says that is not enough.

Ms Shea says it wants a royal commission to determine whether border and maritime officials engaged in criminal activity.

"The royal commission can do things like recommend criminal charges and make policy recommendations, but a Senate inquiry has a narrower scope of powers, so that's why we're asking for a royal commission. And this is a serious thing for Amnesty to ask for. This is not something that we do lightly, and it's not something that we've done before. So we're very worried about what our research has uncovered, and that's why we're asking to get to the bottom of it."

Activist groups have lobbied the government to shut down its detention centres as well as improve its standards at sea.

Peter Dutton was again dismissive of such suggestions.

"People need to understand this message, including the refugee advocates here: We are not going to water down our policy when it comes to boats. We are not going to allow people to settle in Australia if they've sought to come here by boat. We've been clear about it. We've been consistent about it. We're not going to take a backwards step, and we're not going to be bullied into some watering down of that."

Former Salvation Army employee Mark Isaacs spent several weeks working on Nauru in 2012 to provide support for the detainees.

He says, if detaining asylum seekers must continue, an open dialogue with the public is essential.

"I think we need open and transparent systems of detention if we're going to have detention centres. I think we need to allow media to report on detention centres. We need the Australian public to be able to see what's going on inside the centres. I think we need to change the conversation to refer to these people as 'people,' to treat these people as people."

Mr Isaacs says he has seen for himself asylum seekers mentally break down as a result of their time in the centres.

"Pretty much when we arrived there, it was a pretty horrible experience. We saw all sorts of acts of self-harm, of suicide attempts. I saw a man smash a box of fluorescent lights and slash his neck over and over again in front of us. We saw the aftermath of suicide attempts, where I consoled men who had attempted to hang themselves. All these types of things. And then I also saw men lose their minds and have mental breakdowns."

His father, Professor David Isaacs, is a paediatrician and also spent time in Nauru last December caring for children there.

He says what he saw left him shaken.

"I've never seen anything so traumatic in all my life as the children I met there. Just the level of distress in the parents, in the children, in every child I saw, was just awful. And, on average, they had been there 14 months without knowing what was going to happen to them. Very uncertain about the future ..."