Legislation to weaken Television New Zealand's (TVNZ) charter is a way of preparing the state broadcaster for sale, head of the New Zealand Broadcasting School Paul Norris says.

The TVNZ Amendment Bill seeks to replace the charter with a briefer, less prescriptive statement which would enable TVNZ to determine its own priorities.

Mr Norris said despite TVNZ being a state owned enterprise and operating on a commercial model, he saw it was a public asset not a commercial business.

TVNZ needed to have a charter to enshrine its public responsibilities and a system for monitoring those charter commitments, he told Parliament's commerce select committee considering the bill.

A clear policy decision on TVNZs future, including possible sale, should be made before the charter was removed.

"If you remove the charter so TVNZ is left as a fully commercial broadcaster then I can't see any reason why you'd retain it in public ownership," Mr Norris said.

"Therefore sale would have to be not just an option, but probably a preferred option."

Should TVNZ be sold it must be replaced with another form of state broadcasting, he said.

Mr Norris said the charter had problems but programmes, particularly documentaries, had been screened under the charter that would not have been in the late '90s before the charter.

TVNZ executives had a mindset that was so commercial they required a radical jolt to change, he said,