Resignations by MPs has been pronounced since MMP began in 1996, of both of list and constituency MPs.
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 by 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 MP Donna Awatere Huata, was expelled from Parliament.
While at 15 the number of electorate MPs resignations has been higher since MMP, the number of byelections is not much more: eight compared to 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 head the United Nations mission in South Sudan.
Since a law change in 1990, byelections have been avoidable if the resignation is within six months of a general election date announced by the Prime Minister and if 75 per cent of the Parliament agrees.
That happened once before MMP began - in 1996 when Hawkes Bay MP Michael Laws resigned from Parliament in the wake of the Antoinette Beck signature saga.
He had been elected a National MP in 1993 but jumped to New Zealand First during the term.
Since MMP, five byelections have been avoided in that way. But others may be pending.
Prime Minister Bill English has indicated he will follow John Key's practice of announcing the election date early in the year - possibly an announcement as early as February.
National has six sitting electorate MPs who will not be standing next election but could resign earlier under the six-month law, including 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 in known.
Others in the retirement departure lounge are Murray McCully (East Coast Bays), Chester Borrows (Whanganui), Craig Foss (Tukituki), Pseta Sam Lotu Iga (Maungakiekie), and former Labour leader David Cunliffe (New Lynn).
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 who is due to relinquish her portfolio to Nikki Kaye in May; 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 Paul East, Mike Moore, Sir Don McKinnon, Simon Upton, Graham Kelly, Jonathan Hunt, Brian Donnelly, John Carter, and Sir Lockwood Smith.
About half a dozen MPs left under a cloud of some sort, 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 byelection which they contested under their new party banner.
RESIGNATIONS IN 20 YEARS BEFORE MMP
1977 Mangere, Colin Moyle, Labour
1977 Pahiatua, Keith Holyoake, National
1980 Northern Maori, Matiu Rata, ex-Labour
1980 East Coast Bays, Frank Gill, National
1992 Tamaki, Rob Muldoon, National
1992 Wellington Central, Fran Wilde, Labour
1993 Tauranga, Winston Peters, ex National
1994 Selwyn, Ruth Richardson, National
1996 Hawkes Bay, Michael Laws, ex-National
RESIGNATIONS OF ELECTORATE MPS IN 20 YEARS SINCE MMP
1998 Taranaki-King Country, Jim Bolger, National
2004 Te Tai Hauauru, Tariana Turia, ex-Labour
2009 Helen Clark, Mt Albert, Labour
2010 Mana, Winnie Laban, 2010
2011 Botany, Pansy Wong, National, 2011
2011 Te Tai Tokerau, Hone Harawira, ex-Maori Party
2013 Christchurch East, Lianne Dalziel, Labour
2015 Northland, Mike Sabin, National
2016 Mt Roskill, Phil Goff, Labour
2017 Mt Albert, David Shearer, Labour
1999 Waikamiri, Mike Moore, Labour
2008 Rakaia, Brian Connell, National
2011 Northland, John Carter, National
2011 Te Atatu, Chris Carter, Labour
2014 Epsom, John Banks, Act
* Byelection caused by other factors:
* Parekura Horomia's death, Ikaroa Rawhiti, Labour, 2013.
RESIGNATIONS OF LIST MPS IN 20 YEARS SINCE MMP
Jim Gerard, National, 1997
Jill White, Labour 1998
Deborah Morris, ex NZ First, 1998
Paul East, National, 1999
Don McKinnon, National, 2000
Simon Upton, National, 2001
Graham Kelly, Labour, 2003
Jonathan Hunt, Labour, 2005
Jim Sutton, Labour, 2006
Don Brash, National, 2007
Georgina Beyer, Labour, 2007
Brian Donnelly, NZ First, 2008
Ann Hartley, Labour, 2008
Dianne Yates, Labour, 2008
Nandor Tanczos, Greens, 2008
Michael Cullen, Labour, 2009
Richard Worth, National, 2009
Sue Bradford, Greens, 2009
Jeanette Fitzsimons, Greens, 2009
David Garrett, Act, 2010
Darren Hughes, Labour, 2011
Lockwood Smith, National, 2013
Charles Chauvel, Labour, 2013
Jackie Blue, National, 2013
Aaron Gilmore, National, 2013
Katrina Shanks, National, 2014
Shane Jones, Labour, 2014
Russel Norman, Greens, 2015
Tim Groser, National, 2015
Kevin Hague, Greens, 2016
* Vacancies caused by other factors:
*Donna Awatere Huata, Act, (expelled) 2004
*Rod Donald, Greens, (died), 2005
SOURCE: Parliamentary Library