Judith Collins resigned as a minister in August last year, but might soon be back in Cabinet.

Judith Collins will be back in Cabinet by day's end, with the controversial MP set to make a comeback when Prime Minister John Key announces a surprise reshuffle.

Collins resigned as Justice Minister in August last year, amid allegations of inappropriate conduct spawned by Nicky Hager's book Dirty Politics.

But Key is expected to reinstate Collins to Cabinet on Monday afternoon, where it is understood she will pick up the troubled Corrections portfolio she held previously, as well as her former Police portfolio.

Rookie minister Sam Loti-Iiga has struggled in the Corrections role but will likely pick up another portfolio rather than be dumped from Cabinet.

Key has confirmed he will announce his pre-Christmas shake-up at his post-Cabinet press conference, but downplayed the significance.

"It is, I'd emphasise, quite minor - most of Cabinet don't even know so I'm going to tell them this morning," Key told TV3's Paul Henry.

Key said he would not speculate on whether Collins would return to Cabinet or what other ministerial changes were likely.

Of Collins' apparent keenness to regain her ministerial warrant, Key said: "Everybody who's not in Cabinet wants to be in Cabinet, that's the golden rule."

READ MORE: Judith Collins' career timeline

The Corrections Department has faced a multitude of embarrassing headlines this year, including revelations of guards involved in a "fight club" at Mt Eden Prison, and the privately-run prison's slide to the bottom of rankings.

Collins' ministerial resignation, just weeks out from the 2014 election, was prompted by an email, forwarded to the prime minister's office, which appeared to show her involvement in a campaign to undermine the director of the Serious Fraud Office, Adam Feeley.

The email appeared to show that right-wing Whale Oil blogger Cameron Slater had spoken to Collins and was briefing associates on his smear campaign against Feeley, around the time it was suggested the SFO chief had stolen a bottle of champagne in the wake of an investigation into failed finance company Bridgecorp.

In the original email, Slater told his contacts "Collins is gunning for Feeley".

Collins denied it, but resigned, nonetheless. She was later cleared of wrongdoing, and Monday will mark Key's first reshuffle since then.

Collins is no stranger to controversy: revelations about her dealings with New Zealand export company Oravida - of which her husband is a director - dogged both Collins and Key for several months early last year.

Other likely changes include the departure of Trade Minister Tim Groser to become New Zealand's ambassador to the United States, after current ambassador Mike Moore announced last month he was stepping down due to ill health.

ACT leader David Seymour, recently named politician of the year by transTasman, has also been tipped by some to pick up a more significant role within the Government.