Prime Minister John Key will spring a surprise reshuffle today rather than waiting until after the summer holidays.

All eyes will be on whether Judith Collins is returned to a ministerial post. The good money is on her getting back Corrections to sort out an accumulating mess with Serco.

Mr Key will face some serious questions about natural justice if she is not reinstated, even though Ms Collins is a divisive figure within the caucus and National Party.

When questioned on the possibility of Collins returning to cabinet on the Paul Henry Show this morning, Mr Key refused to elaborate.

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She resigned from the Justice portfolio during the election when emails stolen from blogger Cameron Slater implicated her in a campaign to undermine the position of former Serious Fraud Office head Adam Feeley.

Ms Collins was cleared in an inquiry by a retired High Court judge more than a year ago - after Mr Key had picked his third-term Cabinet.

This is his first reshuffle since then.

Ms Collins survived a prolonged controversy earlier last year for helping the Oravida firm in China associated with her husband, but before that she was one of Mr Key's best-performing ministers for almost two terms.

Assuming she is returning to the Cabinet today, she would suit either Corrections or Inland Revenue.

Ms Collins knocked Corrections into shape in the first term of the National Government, and current Corrections Minister Peseta Sam Lotu-liga is struggling with the emerging difficulties of Serco, the private manager of the Mt Eden Correctional Facility.

The departure of Trade Minister Tim Groser to become Ambassador to the United States will be announced today, although his actual resignation may be delayed until February, giving him the chance to sign the Trans Pacific Partnership agreement he has spent the past seven years overseeing.

Mr Groser's Trade portfolio is expected to go to Todd McClay, leaving Inland Revenue vacant.

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While Ms Collins was once tipped as a possible replacement for Mr McClay, more recent speculation has Paul Goldsmith, a minister outside the Cabinet, earmarked for the portfolio.

Other portfolios that could see a switch in minister include Housing, held by Nick Smith, and Workplace Relations and Safety, held by Michael Woodhouse.

Veterans' Affairs Minister Craig Foss could be shuffled out of the ministry altogether.

MPs tipped for promotion include first-termer and Act leader David Seymour and Mark Mitchell, the foreign affairs select committee chair.

Mr Key's reshuffle move today will give him a better summer break and give new ministers a chance to get up to speed on their new portfolios.