In a long-expected move, Senator Brandis will be named as Australia's high commissioner in London, taking over the position vacated by former foreign minister Alexander Downer, when the Prime Minister refreshes his team on Tuesday ahead of a make or break 2018.

Other changes tipped for what could be an expanded, 23-member cabinet include the elevations of Urban Infrastructure Minister Paul Fletcher, Veterans Affairs and Cyber-Security Minister Dan Tehan and the return of former Health Minister Sussan Ley, who resigned over an expenses scandal in January.

New Nationals deputy leader Bridget McKenzie will leap from the backbench straight into the cabinet, possibly in the Regional Communications and Development portfolios vacated by predecessor Fiona Nash.

A third vacancy is expected with Industry, Innovation and Science Minister Arthur Sinodinos – who is battling cancer – to step down.

Fairfax Media understands that Mr Turnbull and deputy Prime Minister Barnaby Joyce discussed the revamped cabinet, which will trigger a slew of further changes to the outer and assistant ministries on Friday and Sunday and were due to do so again on Monday evening.

Mr Joyce is also mulling a switch to the Infrastructure portfolio while the current minister, Darren Chester would switch to Mr Joyce's Agriculture portfolio.

The other cabinet minister said to be in the running for the job of Attorney-General was Employment Minister Michaelia Cash; however, she has been damaged by the recent resignation of a staffer, who tipped off media about a raid on union offices, and accusations she initially misled Parliament over the matter.

Mr Porter's expected from Social Services to Attorney-General – he is a former treasurer and attorney-general in the Western Australian Parliament – will trigger further changes to the cabinet, and the outer ministry.

Senator Brandis was a leading moderate voice in the Turnbull and Abbott cabinets; his resignation will also clear the way for Finance Minister Mathias Cormann to replace him as Coalition Senate leader.

He will exit on a high note, after nearly 18 years in Parliament, having helped lead the case for the historic legalisation of same-sex marriage and having introduced a sweeping package of laws designed to curb foreign interference in Australia's democratic systems and processes.

The shake-up will draw a line under a strong end to what has been a difficult year for the Turnbull government, which has consistently trailed Bill Shorten and Labor by big margins in published opinion polls and endured frequent outbreaks of disunity.

But byelection wins in Bennelong and New England, the referral of two Labor MPs to the High Court over dual citizenship and the resignation of Labor senator Sam Dastyari have boosted government morale.

There have been discussions about Mr Fletcher taking Communications Minister Mitch Fifield's job - and Senator Fifield taking over Social Services - as this would offer a chance to reset debate over the problem-plagued NBN, but such as change is considered unlikely.

Immigration Minister Peter Dutton will also be sworn in to the new super portfolio of Home Affairs, which includes Immigration, the Australian Border Force the Australian Federal Police and spy agency ASIO - the last of which was previously in the Attorney-General's portfolio.

Senator Brandis' Senate seat is likely to be contested by a Melbourne Cup field of candidates, including former Queensland state ministers Lawrence Springborg and Scott Emerson, and former senator Jo Lindgren.

According to the formula used to determine how cabinet portfolios are allocated between the Coalition parties, the Nationals should lose one of their posts – but this is unlikely to happen.

The Prime Minister will also need to fill the outer ministry of Special Minister of State, which was recently vacated by now-Senate president Scott Ryan.

NSW MPs Craig Laundy and Angus Taylor and Queenslanders John McVeigh and Stuart Robert and Karen Andrews are all considered possible promotion candidates for other junior ministry vacancies that will be created.

Also on Tuesday, the Turnbull government will release its long-awaited climate change review.