Teachers who were sacked or quit over alarming conduct remained registered to teach on the official database that is supposed to warn parents and schools.

The Herald on Sunday found three cases of teachers who kept their legal right to work in a classroom despite complaints to the New Zealand Teachers Council. They include:

* A teacher jailed for sex offences against a 13-year-old girl;

* A teacher who quit after an 18-month affair with a student;

* And a principal who quit after allegations he took drugs and paid a student hush money.

The cases have prompted fresh calls from Labour education spokesman Trevor Mallard for a shake-up of the New Zealand Teachers Council, the regulatory and professional body for teachers.

Mallard said there should be an "automatic process" when there was an issue with a teacher. He said that could include a flag on the system that said "this person is under inquiry".

In each case, the teaching registration lasted months beyond schools taking action against their staff.

In the case of Hamilton teacher Jeremy Mark Cavanagh, his registration was still active on Friday despite him being convicted and jailed for sex offences against a teenager.

Former Kawerau College principal Steve Hocking's practising certificate was valid until January this year.

He quit the college in May last year after allegations he took drugs and paid hush money to a former student. The council is considering the case.

And former Putaruru College teacher Ryan Emery, 35, kept his registration until recently even though he quit the school in 2008 after a student said they had been having sex for 18 months.

Principal Mike Ronke said he rang the council only "a few weeks ago" asking why the register showed Emery was still registered and able to teach.

He said he was told the council had an undertaking from Emery he would not seek a teaching position until it had conducted its investigation.

The register now showed his teaching certificate was cancelled.

Teachers' Council director Peter Lind confirmed a safeguard used by the council included an assurance from someone facing official investigation that they would not seek work as teachers.

He said potential new employers usually did checks with previous employers, who could raise concerns.

Lind said Cavanagh was still in jail, which meant there was time for the council to officially deal with issues over his registration.

In other cases, teachers could not be de-registered until all investigations were complete.

In cases where child safety was an issue, the chair of the disciplinary tribunal could ask for interim suspension of a teaching licence.

A new law brought in a month ago means the council can share details with the Ministry of Education's payroll department, so it can track whether teachers under suspicion had found other jobs in the profession.