Troy Buswell will reclaim the role of State Treasurer in a Cabinet reshuffle announced today but the Police Minister Rob Johnson has been dumped from the Ministry.

The Premier Colin Barnett announced the reshuffle after Christian Porter's resignation from Cabinet to make a bid for the federal seat of Pearce.

North Metropolitan Region MLC Michael Mischin will become the new Attorney-General.

Mr Barnett has also used the reshuffle to replace Police Minister Rob Johnson with Scarborough MLA Liza Harvey.

Deputy Premier Kim Hames will replace Mr Johnson as Leader of the House in the Legislative Assembly.

Mr Barnett says Mr Johnson was not happy with the demotion.

Peter Collier will replace Liz Constable as Education Minister.

Ms Constable has previously announced she is retiring at the March election.

In other changes, Murray-Wellington MLA Murray Cowper has been appointed the Minister for Training and Workplace Development and Corrective Services, while Terry Redman will pick up Mr Buswell's Housing portfolio.

The new Ministers will be sworn in on Friday.

Three new parliamentary secretaries have been announced in Riverton MLA Mike Nahan, Jandakot MLA Joe Francis and Ocean Reef MLA Albert Jacob.

Mr Buswell previously held the Treasury portfolio but resigned in April 2010 after admitting using taxpayer funds during an affair with fellow MP, Adele Carles.

After seeking legal advice he later said he did not misuse funds but was not reinstated to the position of Treasurer.

Mr Buswell was later cleared of any wrongdoing in a report from the Public Sector Commissioner, Mal Wauchope

He rejoined Cabinet in December of that year, picking up the portfolios of Housing and Transport.

Political analyst Peter Kennedy says Mr Barnett is hoping the reshuffle will give the Coalition some of the momentum it lost when Mark McGowan replaced Eric Ripper as leader of the Labor Party.

"By giving the team a new look, he hopes that'll freshen the team up, give it a new look and give it a bit more vigour going into the election and obviously hopes that that will help the government regain the momentum that it lost a few months ago after Mr McGowan became the Labor leader."

Mr Kennedy says he is not surprised that Mr Johnson has been dumped from the Ministry after recent controversies.

"Probably Mr Barnett would have been criticised if he retained Rob Johnson," he said.

"He's a senior politician and Mr Barnett's looking for youth, he's looking for new faces and I think that's the reason Mr Johnson has been a casualty."

However, Mr Barnett has left the door open for Mr Johnson to become Speaker, if he stays on and the Government wins the next election.

"Rob would like to be Speaker - I don't think there is any secret about that - but there is no agreement or deal as such, and if we win, I can't say who would be Speaker but I certainly know he has an interest in that role."