Former Prime Minister John Key is back at Parliament - for a limited time.

Key slipped into his new office in Parliament this morning and National's caucus meeting without detection, but his cover was blown when his new boss Bill English tweeted a photo of himself popping in to visit Key.

It is the first day back for Parliament for the year and is likely to be Key's final six weeks as an MP.

Key can leave within six months of the September 23 election without forcing a byelection in his Helensville seat. He will be joined in his exodus by the man he beat in the 2014 election, former Labour leader David Cunliffe.

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Cunliffe has also been waiting for the six-month deadline to click over to leave for a consulting job without triggering a byelection in New Lynn.

While English picked up Key's old schedule - a trip to Europe, the visit to Ratana, a State of the Nation speech and Waitangi Day weekend visits, Key has been working on his golf on holiday in Maui and in New Zealand.

He has reported he went well under his handicap of 11 at the Kauri Cliffs golf course recently.