Resignations by both list and constituency MPs have been pronounced since MMP began in 1996.
In the 20 years before MMP began in 1996, there were 14 vacancies - nine caused by resignations of MPs and six by deaths while in office.
In the 20 years since MMP there have been 48 vacancies, 30 of which have been from list MPs resigning, according to data from the Parliamentary Library.
In such cases, the list MPs were replaced by the next available candidate on the party list.
Most of the other vacancies were created by the resignation of 15 constituency MPs, more than the nine resignations in the 20 years before MMP.
Two vacancies since MMP were created by deaths of MPs, Green co-leader Rod Donald and Labour MP Parekura Horomia.
One MP, Act New Zealand's Donna Awatere Huata, was expelled from Parliament.
While at 15, the number of electorate MP resignations has been higher since MMP, the number of by-elections is not much more: eight compared with 10, forced by resignations.
The 10 includes the next one, to be held in Mt Albert on February 25 following David Shearer's resignation to take up a UN role.
Since a law change in 1990, by-elections have been avoidable if the resignation is within six months of a general election date announced by the Prime Minister and if 75% of the Parliament agrees.
That happened once before MMP began - in 1996 when Hawke's Bay MP Michael Laws resigned from Parliament following the Antoinette Beck signature saga.
Since MMP, five by-elections have been avoided in that way. But others may be pending.
National has six sitting electorate MPs who will not be standing next election but could resign earlier under the six-month law, including John Key himself, the MP for Helensville.
Maurice Williamson, the MP for Pakuranga, is set to resign early to take up a posting as New Zealand's next Consul-general in Los Angeles, once an election date is known.
Others in the retirement departure lounge are Murray McCully (East Coast Bays), Chester Borrows (Whanganui), Craig Foss (Tukituki), and Peseta Sam Lotu Iiga (Maungakiekie).
At least five list MPs from different parties have given notice of their intention to retire and could leave before the election. They include: Education Minister Hekia Parata, Labour MP Clayton Cosgrove and Green MPs Catherine Delahunty and Steffan Browning.
About 12 of the total 48 departures since MMP were by MPs going to diplomatic postings or elected international positions, including Mike Moore, Sir Don McKinnon and Sir Lockwood Smith.
About half a dozen MPs left under a cloud, including Richard Worth, Pansy Wong, Darren Hughes, Aaron Gilmore, John Banks and Mike Sabin.
Two MPs, Tariana Turia and Hone Harawira, resigned from their parties and from Parliament to force a by-election, which they contested under their new party banner.
