The United Nations high commissioner for refugees has rejected Peter Dutton’s suggestion that white South African farmers should receive “special attention” for migration to Australia.

At a press conference in Canberra to warn of the “devastating” impact of detention on children, the UNHCR director for Asia and the Pacific, Indrika Ratwatte, said refugees on Nauru should be prioritised and quotas should not be spent on any decision to open migration to South African farmers.

On Tuesday seven backbench MPs raised the plight of white South African farmers in the Coalition party room, and the response from Dutton confirmed Australia could issue humanitarian visas even if they didn’t qualify as refugees.

Ratwatte has returned from an inspection of facilities on Nauru, making him the most senior UNHCR official to witness Australia’s offshore processing facilities first-hand.

On Tuesday Ratwatte described the mental health of people he saw as “very, very shocking” and said the growing trend to self-harm and children as young as 10 attempting suicide was “symptomatic of the despair and hopelessness prevailing there”.

He noted that 40 children on Nauru had spent their entire lives in detention and another 60 had spent half their lives, four years or more, in detention.

Ratwatte warned that keeping refugees on Nauru was leading to family separation, citing the example of a 14-year-old girl on Nauru separated from her mother and siblings receiving medical attention in Australia.

“This little girl was in a catatonic state – she hasn’t gotten out of her room in months – who has not taken a shower and is in a state of complete stress and trauma.”

[Places] should be preserved for those who are truly vulnerable and in need of protection. Indrika Ratwatte, UNHCR

He said that while it was encouraging the United States is taking a “significant number” of refugees, there will still be 1,000 refugees in need of resettlement solutions to leave detention.

In recent weeks the home affairs minister, Peter Dutton, and the former prime minister Tony Abbott have called for white South African farmers to receive “special attention” due to alleged elevated rates of violence against them associated with possible land redistribution.

Ratwatte responded: “The decision of the government to open its migration pathways to different categories of people is a sovereign decision but from the UNHCR perspective we do encourage that resettlement opportunities that are for refugees and humanitarian quotas, that are for deserving cases, should not be impacted by these decisions on migration.

“[Places] should be preserved for those who are truly vulnerable and in need of protection and a solution.”

Seven backbench MPs – including Andrew Hastie, Andrew Laming, Craig Kelly, Jim Molan and Luke Howarth – raised the issue in the party room, warning of high rates of violence in South Africa and defending Dutton against charges of racism.

Concerns were expressed white South African farmers may not qualify as refugees, even if they left South Africa, because of the possibility they could move to cities in country and be free of persecution. But the party room was assured Australia could still grant humanitarian visas. Applications, including from referrals in Australia, will be considered.

Asked if the people on Nauru were more deserving, Ratwatte said the “facts speak for themselves” and it was self-evident from the trauma and vulnerability of those on Nauru – more than 80% of who have been assessed as refugees – that they are deserving of “prioritised solutions”.

Ratwatte refuted claims that refugees and asylum seekers engaged in self-harm to transfer to Australia, saying it was “not the case”.

“Nobody should be in a position to seek such dire recourse – for children in particular, swallowing razors and taking medication overdoses is not what this is all about. They should have safety and protection.”

He noted the Australian government’s position that offshore detention was needed to prevent deaths at sea but said it had a responsibility to ensure the policy “does not lead to individuals eventually being harmed on land”.

Malcolm Turnbull has so far refused New Zealand’s offer to accept 150 refugees from Manus Island although he has suggested it could be considered after the US resettlement deal.

After Dutton’s comments, several Turnbull government ministers including Julie Bishop and Mathias Cormann have publicly stressed the non-discriminatory nature of the humanitarian visa program.

Despite the apparent contradiction, Bishop told the party room stressed the consistency of her stance with Dutton’s because both agree that South Africans would qualify under existing humanitarian visa rules.