The federal government should scrap a planned plebiscite on same-sex marriage and divert the $160 million in savings to Syria as humanitarian assistance, Labor says.
Foreign Minister Julie Bishop is expected to announce $25 million in additional Australian aid at a Syria Donors Conference in London on Thursday.
Opposition foreign affairs spokeswoman Tanya Plibersek says that amount is not enough.
The $25 million was "extraordinarily low" compared to the contributions of other countries, she said.
Aid group Oxfam had calculated that a fairer share would be $178 million.
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"We could maybe take the $160 million we're spending on this dumb plebiscite and do something useful with it," Ms Plibersek told reporters in Canberra.
Oxfam chief executive Helen Szoke urged Ms Bishop to "lead with her heart" and up the ante for other donors at the conference.
"Even if all of the $25 million went to respond to the Syria crisis, Australia would only be meeting 14 per cent of its fair share of funding for the crisis for this year," she said in a statement.
