An exclusive private school run by an "extremist cult" that warns children to stay away from the outside world and bans its graduates from physically attending university receives more in government funding per student than up to a third of the state's public schools.

Data obtained by Fairfax Media from the MySchool website reveals that a school run by Protestant religious sect, the Exclusive Brethren, receives $800 more in public funding per student than Homebush West, which has been forced to ban children from running in its playground due to overcrowding.

At the same time, the school receives just $3 less in public funding per student than one of the state's poorer public schools, Liverpool Public.

The Liverpool school has a significantly lower ICSEA, or socio-educational advantage rating compared to the Brethren's MET school in western Sydney and receives $10 million less in donations every year.

The religious group, which has 15,000 members in Australia and 40,000 worldwide has been in the spotlight since June after Fairfax Media revealed the group had covered up instances of alleged child sexual abuse.

Brethren members are prohibited from eating or socialising with "worldly" people, and those who leave the church are banned from seeing their families, including their own children.

The sect first made headlines on the political scene after it funnelled hundreds of thousands of dollars into former prime minister John Howard's re-election campaign in 2004, and was later referred to in Liberal Party documents as "friends."

After winning office in 2007, Labor leader Kevin Rudd described the group as "an extremist cult that breaks up families," but continued the pattern of increasing funding to its schools.

In total, more than 600 public schools across the state receive less public funding per student than the Brethren institution, which has been rebranded as part of the Plymouth Brethren Christian Church.

Successive NSW and federal governments have continued to fund the 1000-student, 11-campus school at record levels of more than $10,000 per student despite parents and brethren businesses bolstering its coffers through up to $10 million in gifts every year - three times the amount of income it receives in school fees.

A ruling from the Australian Tax Office in 2005 found that offsetting fees through tax free donations would be illegal.

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In a statement through public relations firm Wells Haslem, the church said it has never accepted donations in lieu of school fees and that comparing private school funding with public school funding was "misleading and irrelevant."

"The way government schools and non-government schools are funded is quite different and can't be compared," the church said.

In defending its income stream, the school said "as a relatively new school" it had spent more than $4 million on facilities in 2014. The Brethren school was established 23 years ago.

A former Brethren school principal, David Stewart, said in 2007 that the sect's school system had been set up "to prevent the children from being corrupted" by things such as reading novels, and world leader Bruce Hales said in 2004 that the school system, which "the government has given us", would "deliver the young people from the world". 

Private schools that receive a similar level of public funding, such as Emmaus Catholic College in Kemps Creek, have one-twentieth the level of private donations.

The combination of a high level of donations and public funding has meant that the Brethren school has been able to guarantee funding of up to $22,000, per student per year, more than many private schools secure through student fees, according to MySchool data.

Within its own postcode, the Brethren school outstrips public funding per student for two public schools, Burnside and Parramatta East.

Its school funding platform has drawn criticism for more than a decade, but taxpayer payments have continued to increase at record levels.

Recently re-elected federal senator Nick Xenophon called for an investigation in 2011 after describing the arrangement as a "tax lurk of biblical proportions".

In June, Prime Minister Malcolm Turnbull said he had "no criticisms or complaints" to make of the controversial Brethren and was happy for the extremist Christian sect to continue donating to the Liberal Party.

The comments came after it was revealed its leader, Bruce "the Elect" Hales, who has been known to transport himself in a private jet, advised a young follower to "finish yourself off" with arsenic rather than communicate with excommunicated members of his own family.

In a statement, the NSW Department of Education could not explain why the school was receiving funding equivalent to that of some of the state's poorest public schools despite having a donation base five hundred times their value.

"State funding levels recognise the level of resources available to each school based on fees, charges and parent contributions. Schools must be registered and not operate for profit to be eligible for state funding," a spokesman said.

The federal Department of Education has refused to answer how the Brethren school received more public funding than other comparable schools despite receiving $10 million in donations per year.

A spokesman for the department said funding was allocated on a number of factors including the number of students from a lower socioeconomic background, who have a disability, are of Aboriginal and Torres Strait Islander background, have a low English proficiency, and the school's size or location.

But MySchool data shows the MET school has no Aboriginal students, no students of a language background other than English and an average socioeconomic background.

The department would not comment on whether it had any plans to investigate the funding arrangement.