The Prime Minister is defending the use of taxpayer-funded travel and accommodation for ministers campaigning for the National Party candidate in Northland.
Ministers have discretion over the use of their Crown cars but there is a general principle that politicians do not use taxpayer funds for electioneering or party business.
During the Labour leadership campaign last year, flights for the three candidates were paid for by the taxpayer, but other spending - including taxis and phone calls - was covered by the MPs themselves.
Labour defended the cost of the flights, saying it was a legitimate parliamentary expense.
Leader Andrew Little said the difference here was National was inundating the electorate with its MPs.
"When you've got Cabinet ministers by the dozen and National MPs by the baker's dozen going up there finding places to build bridges, no, I think it goes back to them. They are desperate."
The Greens said if ministers were electioneering to promote their own party in the Northland by-election, they should foot the bill.
"It's pretty extraordinary that National's not only pork-barrelling the electorate but accessing the pork themselves to get themselves around the electorate," Green Party co-leader Russel Norman said.
"It doesn't seem right that there's one rule for those who are trying to beat National, and a different rule for the National Party themselves."
Mr Key said the use of taxpayer-funded travel and accommodation was within the rules.
"If a by-election is a legitimate point around democracy for the people of Northland, and we need to stick within the rules, and we stick absolutely religiously to those rules."
Finance Minister Bill English said he would use a Crown car when up in Northland, as long as it was within the rules.
His colleague Paula Bennett said she would be using private transport, because the main purpose of her being in the electorate was to support the National candidate.