The Australian and international news stories you need to know today, Wednesday October 28.

NSW reported two new cases of locally transmitted COVID-19, while there were 10 new cases in returned travellers in hotel quarantine.

Thousands of children at more than 20 Sydney schools have been forced out of classes after threatening emails were received while students sat HSC exams.

A high school on the lower north shore was the first to receive threats about 11am on Tuesday, via an anonymous email.

Since then, police say at least another 18 high schools across Sydney received similar threatening emails.

DEVELOPING: A major police investigation is underway after threats were sent to at least 19 schools across Sydney, forcing them to be evacuated. https://t.co/LZzdqRXpfn @robertovadia #7NEWS pic.twitter.com/Mmwv0cruap

— 7NEWS Sydney (@7NewsSydney) October 27, 2020

The schools all enacted their emergency response plans, evacuated students, and contacted police.

Local and specialist officers are searching the schools, but no items of interest had been found by the end of the school day, police say.

The threats caused thousands of students sitting their HSC exams to be pulled from classrooms on one of the most important days of their schooling lives.

Many shattered students said they didn't get to finish their tests.

All the campuses will resume normal operations on Wednesday.

People recovering from COVID-19 may suffer significant brain function impacts, with the worst cases of the infection linked to mental decline equivalent to the brain ageing by 10 years, researchers have warned.

A non-peer-reviewed study of more than 84,000 people, led by Adam Hampshire, a doctor at Imperial College London, found that in some severe cases, coronavirus infection is linked to substantial cognitive deficits for months.