We must be relentless against terror: PM

Prime Minister Malcolm Turnbull has declared terrorists should face the "full severity of the law" a day after a Melbourne teenager was sentenced to seven years in jail for preparing a bomb attack.

A Victorian Supreme Court judge on Wednesday said the 18-year-old, who cannot be named, had "every intention" of using the partially constructed pipe bombs found in his home and the only reason he didn't go through with his plot was that he was arrested.

Mr Turnbull on Thursday was asked if he thought the sentence was too lenient.

The prime minister replied that sentencing was a matter for the courts and he wouldn't comment on any particular case.

"(But) terrorist offences should be dealt with with the full severity of the law," he told 3AW.

"We need to be relentless in stating our absolute condemnation of terrorism and terrorist activities and people who support terrorist activities."

Mr Turnbull said it was up to prosecutors to appeal if they thought any sentence was too light.

"They've got the right to do so and where sentences are seen to be inadequate then they should do so."

The prime minister added new counter-terrorism laws meant serious offenders could be kept in jail after their sentence ended "if a court concludes they have not been rehabilitated".

The devices the teen was making were similar to those that killed and maimed the victims of the Boston Marathon terror attacks, according to the FBI.

The youth will be eligible for parole after serving five years and three months, and has already spent more than 570 days in custody.

Victorian Opposition Leader Matthew Guy doesn't think the seven-year term is long enough and he was happy to comment on Thursday.

"People need to be serious about this, the courts need to be serious about this," Mr Guy told reporters.

"The public is frightened, the public is concerned. We keep seeing weak sentence after weak sentence."