The Tasmanian Government's bill for a minimum mandatory penalty for assaults on police officers has passed Parliament.

The bill means serious assaults on police will carry a minimum six-month jail term.

The Police Association's Pat Allen said a violent incident on Hobart's waterfront four years ago highlighted the need for the sentences.

"[The offender] walked around and punched the male police officer in the face, while his arms were folded and he wasn't looking in his direction, he nearly put him through a glass door," he said.

"He [the officer] had ongoing issues for at least three, four months.

"He has ongoing issues now.

"That person [the offender] didn't even get jail for it."

Mr Allen said similar legislation in Western Australia had reduced assaults on police.

"There is as much evidence that it works as there is saying that it doesn't work," he said.

The legislation includes an exceptional circumstances clause but Matthew Verney from the Law Society said it is not flexible enough.

"There's too much uncertainty about that, and that's why we were always opposed to that sort of provision," he said.

Mr Verney said mandatory minimum penalties are misguided in removing judicial discretion.

"There've been 13 matters before the Supreme Court under the relevant criminal code provision, 11 of those have received immediate custodial sentences," he said.

"That's as a result of the court properly taking account of all of the relevant circumstances.

"The court now won't have that opportunity."