Danish MPs have dropped a planned visit to Australia's immigration detention centre on Nauru after two politicians who criticised the centre were denied access, a delegation member says.

Key points: Two left wing politicians were denied access to Nauru

Two left wing politicians were denied access to Nauru MP Johanne Schmidt-Nielsen calls the decision "unacceptable and anti-democratic"

MP Johanne Schmidt-Nielsen calls the decision "unacceptable and anti-democratic" Red-Green Alliance adviser initially expressed surprise that visit was given green light

Johanne Schmidt-Nielsen of the leftist Red-Green Alliance and Jacob Mark of the Socialist People's Party had both criticised Canberra's use of offshore settlements for asylum seekers and conditions at the Nauru centre before travelling to Australia last week.

"After the Danish Foreign Ministry got involved in the case, Nauru authorities said on Tuesday that the decision [to deny access] remained in force," Ms Schmidt-Nielsen wrote on Facebook.

A Danish Foreign Ministry spokesman confirmed the information.

The entire delegation then decided to cancel its visit to the island, even though MPs from the Social Democrats, the centre-right Venstre party and the anti-immigration Danish People's Party would have been able to go.

"It is absolutely unacceptable and anti-democratic and says everything about the situation in Nauru that critics are denied access," Ms Schmidt-Nielsen added.

A third politician who had not voiced any criticism against the Nauru immigration centre — Naser Khader of the Conservative People's Party — was also denied access.

While Danish media speculated that his application to join the tour may have come too late, Ms Schmidt-Nielsen suggested that his Arab name may have played a part in the decision.

Red-Green Alliance planned to be 'eyes and ears of world'

Adviser to the Red-Green Alliance Niels Rohleder told the ABC's Pacific Beat program last week that visas for the group were originally organised by the Danish embassy in Canberra.

Sorry, this audio has expired Danish MPs to visit Nauru for offshore resettlement study ( Ian Burrows )

"I, myself, was surprised that the group of six parliamentarians were going to be allowed into Nauru," he said.

"As I understand it, it is extremely difficult for others to get access. But I'm glad it's been made possible."

Mr Rohleder said he believed the study trip, which was being pushed by a member of the Danish People's Party, was not relevant to Denmark's situation but that the Red-Green Alliance would make the most of it nonetheless.

"It's important for us to be there ... to act as the eyes and ears of the world in a situation where most Australian and international journalists are actually barred from going to the island because of the visa restrictions," he said.

Ms Schmidt-Nielsen had said that she would use the trip to ask "critical questions".

AFP/ABC