Tony Abbott has accused the United Nations of trusting the claims of “the usual human rights activists” to make “absolutely bizarre” findings about Australia’s treatment of asylum seekers.

The prime minister said the UN special rapporteur on torture, Juan Mendez, “didn’t bother getting a proper response from the Australian government” – even though the findings published on Monday referred to correspondence with the government and the inadequacy of the official responses.

Labor accused the Coalition of “shooting the messenger” and of failing to provide sufficient answers to the UN’s concerns.

In a report delivered to the UN human rights council, Mendez found Australia was violating the international Convention Against Torture by detaining children in immigration detention, and holding asylum seekers in dangerous and violent conditions on Manus Island.

Abbott stepped up his criticism of the findings on Wednesday, adding to the prime minister’s previous comments that Australians were “sick of being lectured to by the United Nations”.

“I think torture no less was what we were accused of inflicting on people,” Abbott told 2GB in an interview.

“Well, I mean, this is really bizarre – absolutely bizarre. You’d think the UN would be saying good on Australia for stopping the boats and stopping the drowning.”

The interviewer, conservative broadcaster Alan Jones, said that Abbott had “got the socialists upset”. Abbott then criticised the process Mendez followed in investigating allegations.

“It seems that the UN rapporteur didn’t even bother getting a response from the Australian government – or didn’t bother getting a proper response from the Australian government; went to the usual suspects, the usual human rights activists, accepted everything that they said as gospel truth and now we’ve got what is supposed to be a reputable body criticising the Australian government for doing the right thing,” Abbott said.

But the report shows Australia had significant correspondence with Mendez about his questions and concerns. The government wrote to his office at least four times between May and December 2014.

Mendez raised four specific issues with Australia.

Addressing three of those issues – conditions in immigration detention, holding of asylum seekers incommunicado at sea, and the alleged maltreatment of asylum seekers on Manus Island – Mendez thanked the government for its responses, but found “that the government, in its reply, does not sufficiently address the concerns, legal obligations, and questions raised in the initial communication”.

Mendez wrote that, as a result, he must infer that “the government fails to fully and expeditiously co-operate … as well as to comply with its obligation, under international customary law, to investigate, prosecute and punish all acts of torture and other cruel, inhuman or degrading treatment or punishment”.

In one of the cases he found there was substance in the allegations “in the absence of sufficient information to the contrary”.

When dealing with a fourth issue – changes to migration laws that passed the parliament last year – Mendez acknowledged the government’s response on 23 December was “comprehensive” but he still surmised “that both bills put Australia at risk of violating the Convention Against Torture”.

He found that the migration and maritime powers legislation amendment violated the convention “because it allows for the arbitrary detention and refugee determination at sea, without access to lawyers”.

The migration amendment (character and general visa cancellation) bill was in contravention “because it tightens control on the issuance of visas on the basis of character and risk assessments”.

“The rapporteur concludes that the government of Australia, by failing to amend the provisions of the two bills to comply with the state’s obligations under international human rights law, particularly with regard to the rights of migrants, and asylum seekers, including children, has violated the rights of migrants and asylum seekers to be free from torture or cruel, inhuman or degrading treatment, as provided by articles 1, 3, and 16 of the [convention],” the report said.

Labor’s immigration spokesman, Richard Marles, said the government had treated an important process with contempt by providing inadequate replies to Mendez.

“It’s not good enough to just shoot the messenger,” Marles told Sky News. The previous Labor government reopened the Manus Island detention centre in 2012.

Mendez defended his report after Abbott’s initial comments about the UN “lecturing” Australia.

“I’m hoping that we can have a more direct dialogue to clarify what I think are appropriate responses and that we don’t engage in a dialogue only through the media because that tends to result in intemperate responses like the one you mentioned by prime minister Abbott,” Mendez told the ABC on Tuesday.

“But I do want to say that my office and me, we are absolutely willing to engage with the Australian government at any level and any moment, privately or publicly, because my intention is to help Australia live up to its obligations under international law.”