UPDATED 6.19PM A date's been set for a by-election in the Labour held seat of Mount Roskill.

The seat is to be left vacant with sitting MP Phil Goff set to resign from Parliament this week to take up his role as Auckland's newly elected mayor.

The exact timing of the by-election will be driven by the timing of his resignation, but the Prime Minister has a date in mind.

"On the basis that Mr Goff resigns this week, it is my intention that the by-election will be held on Saturday December the 3rd."

John Key has all but confirmed a Cabinet reshuffle in the New Year, and said he's expecting a few ministers to step down ahead of the general election.

The Prime Minister won't say which ministers might be moving on although there's pressure on Nick Smith, who's been criticised on housing and other issues.