The Coalition and Labor will back a plan to divide up six and three-year Senate terms that benefits them but threatens to antagonise key crossbenchers, whose time in Parliament may be cut short as a result.

Senators are usually elected for six-year terms with half up for election every three years. Following a double dissolution election, which clears all seats, the Senate is empowered to decide how six and three-year terms are assigned.

The "order of election" method being backed by the government and opposition sees the first six senators elected in each state serve the longer term while the bottom six are relegated and forced to seek re-election in 2019.

"12 senators are elected in every state. Under the methodology used on every occasion so far, the first six senators elected would get six years, whoever they are and whichever party they represent," Finance Minister and deputy government leader in the Senate Mathias Cormann said on Friday.