One in 30 of the state's International Baccalaureate students received the perfect score of 45, which translates to the top ATAR of 99.95, when results were released on Sunday.

By comparison, fewer than one in 1000 HSC students received 99.95 last month.

However, while the HSC serves 70,000 students across the state, the International Baccalaureate (IB) is only offered as an alternative at about 15 private schools in Sydney, with 346 students receiving results on Sunday.

Denice Scala, the principal at MLC School in Burwood where one student received 45 and six received 44, said there was nothing to be gained by comparing the IB with the HSC.

"Instead, we should focus on providing students with the freedom to choose," she said. "The most important thing is choosing the option which suits a student's learning style, individual strengths and matriculation preferences."

NSW is the only state in Australia that does not allow public schools to offer the IB.

While the HSC allows students to specialise in an area of study, IB candidates must study six subjects including English, a second language, maths, a science subject and a humanities subject. They must also complete a 4000-word research essay, study a subject called the theory of knowledge and undertake community service.

Of the 1790 Australian students who completed their diploma in November, 31 scored the top mark of 45, with 12 of those students graduating from schools in Sydney.

Despite making up only a fraction of the 1.2 million students from 141 countries, Australians made up more than 10 per cent of the students awarded 45s.

The average score among Australian students was 33.96, which is equivalent to an ATAR of 91.45, and the pass rate was 92.66 per cent. The global average of about 30 would translate to an ATAR of 83.

However, unlike NSW, in most major IB countries the qualification is offered predominantly at public schools.

At Queenwood School for Girls in Mosman, two students scored 45, and 13 of the 15 candidates received ATARs above 90.

Three students from Newington College, one student from Monte Sant' Angelo Mercy College, one student from Redlands and one student from St Paul's Grammar also received 45.

The average ATAR at St Andrew's Cathedral School was above 90, with the top student receiving a 41.

The students from MLC School who gathered on Sunday to share their results largely exceeded their expectations.

"I was ecstatic. I honestly didn't expect it, I really didn't. I would go over my marks and predict my score and I expected 38 maybe." Amaani Hatoum, who received a 44, or an ATAR of 99.85, said. "Girls tend to lower their expectations so they don't get hurt or they're not disappointed."

More than half of all the HSC and IB students at MLC School received an ATAR of more than 90.

Megan Foo, who wants to study medicine, said she had a nightmare in the lead-up to Sunday that her mark would be 36.

"I woke up sweating, so the 43 was quite a relief for me," she said.