Oxfam at Work, the biggest campaign of its kind, is being promoted through the national press, online banner advertising and a dedicated section of the charity's website as "the easiest, most tax-efficient way of giving money to help end poverty".
A spokeswoman said: "The campaign was planned prior to the Budget, but the tax cut has given the issue prominence. £1.8m is equal to what we spend in two years in Rwanda."
Payroll-giving schemes allow employees to donate from their gross salaries, which means that charities can collect the tax that would otherwise go to the Treasury. As a result of the cut in basic income tax, next year charities will be able to reclaim only 25p from each £1 donated, rather than the 28p they receive now.
Tina Steele, payroll giving project manager at the Institute of Fundraising, said all charities would benefit from Oxfam's new campaign.
"People who have never thought about payroll giving before will take action, even if they don't want to give to Oxfam," she said.
Institute figures show that payroll-giving schemes raised more than £84m for UK charities during 2005/06.