A 51-year-old married teacher who struck up a relationship with a 16-year-old schoolgirl could face dismissal and blacklisting by education authorities.

Jeff Sinclair, who teaches English at a Sydney high school, is being investigated by the New South Wales Education Department over his affair with Nicki Shackle, now aged 18.

The department will make public its report within the next few weeks and has the power to recommend Sinclair be sacked, demoted or blacklisted, blocking future employment.

The pair began their relationship more than two years ago, when Shackle was a pupil in year 10 at Baulkham Hills High, in Sydney's northwestern suburbs.

The couple, who now live together in a Sydney apartment, insist they did not have sex until Shackle turned 16. Police investigated the case but could not find enough evidence to bring a charge of statutory rape.

The case has attracted a huge amount of media interest after Shackle's parents, Steve and Judy, went public with the affair last month.

Shackle says she has been ostracised by her parents, who have stopped her from seeing her 15-year-old brother and 13-year-old sister unless the children are accompanied.

In an interview with the Sun Herald newspaper at the weekend, Shackle, who is now studying for an arts degree at Sydney University, said she initially bonded with Sinclair over a shared love of literature, music and poetry.

"We just had similar interests and loved to talk to each other," she said.

"There was never any impropriety in our relationship at all, and we have never done anything wrong. Our relationship has never been abusive in any sense of the word ... "If the [authorities] want to catch a paedophile, they're looking in the wrong place."

The Education Department initiated an inquiry into Sinclair two years ago, following a formal complaint from Shackle's parents. He has been moved to administrative duties pending the completion of the inquiry.

After Shackle moved out of the family home in 2001, her parents started writing letters to ministers, Government officials and welfare groups.

In one letter, Judy Shackle, who is almost 10 years younger than her daughter's lover, warned the school of an "inappropriate relationship" developing between our then 15-year-old daughter and a 50-year-old married teacher".

"We expected the school to rally behind a close-knit, loving family, but instead we were ostracised, a school cover-up initiated and all communications from the school ceased," she wrote.