An Australian teacher who gave English lessons to asylum seeker children on Nauru has returned home with a suspected brain impairment after living and working in buildings "alive" with mould.

The woman, who asked not to be named, is one of an estimated 20 former Nauru immigration detention centre workers who have become seriously ill because of exposure to mould at the facility.

"The walls were alive," she told AAP, as she recalled seeing workers wearing full plastic suits carrying out rectification work in the modular buildings at the staff accommodation.

"I've got a cognitive disability from my mould inhalation," she said.

The speech and memory impacts have put an end to her teaching career after she developed toxic mould syndrome and suffered ongoing fatigue.

"Often I can't think of the word I'm trying to say."

"I'm not as outgoing as I used to be. I'm a completely different person.

"I feel discarded and pretty useless."

A top-secret report from December 2014 warned Australia's immigration department (now Home Affairs) and then-detention centre operator Transfield (now Broadspectrum) of mould health risks.

Another secret report from June 2015 warned Transfield's mould remediation plan was inadequate and did not represent international best practice.

Fibromyalgia, sarcoidosis, neurological symptoms, chronic pain, chest infections and persistent coughs, are among the medical problems some ex-staff have developed.

Mould exposure also exacerbated multiple asthma and sinusitis cases among asylum seekers, according to Doctors for Refugees.

One youth worker told AAP centre management had forced a group of 14 to 17-year-old boys to clean off graffiti scratched onto their mouldy tent without any face masks or rubber gloves.

A 15-month AAP investigation revealed construction and mould problems at Nauru and Manus Island detention centres in taxpayer-funded staff accommodation built from 2013 onwards.

Former Save the Children caseworker Steve Fisher, who worked on Nauru from mid-2014 to the end of 2015, said he had a mysterious persistent cough for years that doctors had been unable to diagnose because X-ray results were clear.

"It feels like there's a foot on my chest," he told AAP.

Authorities had frequently reassured staff there were no health risks from the mould, Mr Fisher said.

AAP has obtained a 247-page report Transfield commissioned by microbiologist Dr Cameron Jones from Biological Health Services who visited Nauru in October 2014.

It warned airborne and surface-bound fungal and bacterial cells and spores are capable of causing disease "by direct infection, toxicosis or by allergy".

Dr Jones heard about "unverified reports of illness" caused by mould but didn't have time to meet with the centre's health clinic staff.

He warned pregnant women, infants under two, people with compromised immune systems, asthmatics and people with allergies should be removed from contaminated environments and during remediation.

"Failing to inform or evacuate persons with these conditions may lead to a serious breach of duty of care," the report said.

Previously, Transfield had been trying to kill mould with bleach but it grew back after several weeks.

The Jones report said scientific tests found all accommodation tents showed more than 10 square metres of mould and should be thrown out and replaced.

The immigration department commissioned Brian Murphy from Greencap to review the Jones report and Transfield's mould remediation plan in June 2015.

Mr Murphy wrote Transfield's plan is "highly unlikely to adequately rectify and prevent the re-occurrence of the extensive moisture and mould issues" and lacked an understanding of "internationally accepted best practices."

Another mould expert Jeremy Stamkos from Eronmor, was sent to Nauru in April 2015 at the request of building contractor Canstruct.

"The presence of wet and mouldy building materials may present a risk to occupant health," Mr Stamkos wrote in his report.

Rooms and buildings with on-going moisture dampness, strong musty odours and visible mould "may not be suitable for occupancy", Mr Stamkos said.

The report attributed the moisture problems to air conditioning running at below 22 degrees which caused condensation.

Inspector reports from work health and safety regulator Comcare acknowledged the mould problems but gave conditions the nod during visits in 2014, 2015 and 2017.

The Home Affairs Department denied there had been any cases of illnesses as a result of mould exposure at the Nauru detention centre.

"There is no health risk to anyone at the Nauru Regional Processing Centre relating to mould and the department strongly disputes any claim to the contrary," a spokesman said.

However, Comcare told AAP it received a complaint in 2016 from a worker employed by a contractor, who had been diagnosed with a respiratory condition after living at the Nauru detention centre.

"We are satisfied that reasonably practicable steps were taken to address problems with mould at the centre, and that the most recent site inspection in August 2017 found no significant issues," Comcare said in a statement.

Canstruct defended its construction work saying "no corners were cut at any stage", while Broadspectrum said it maintained a safe and healthy workplace for employees at the detention centre.

MOULD RATINGS IN 2014 JONES REPORT

ASYLUM SEEKER TENTS

Tent #37 single males: Mould rating: four (visible mould growth greater than 10 sq m)

Tent #39 family: Mould rating four (visible mould growth greater than 10 sq m)

STAFF ACCOMMODATION

Women's unit #103: Mould rating five (strong indication of mould but no visible evidence)

Men's unit #219: Mould rating five (strong indication of mould but no visible evidence)

MAIN ADMINISTRATION AREA

Mould rating four (visible mould growth greater than 10 sq m)

PSYCARE OFFICES

Office 2: Mould rating three (visible mould growth from one sq m-10 sq m)

MESS HALL

Mould rating four (visible mould growth greater than 10m2 which has damaged the structural integrity of wall linings and ceilings.)

COLD STORAGE

Mould rating four (visible mould growth greater than 10 sq m which has damaged the structural integrity of wall linings and ceilings.)

SCHOOL BUILDING

Mould rating two (visible mould growth to less than one sq m)