She says the government introduced the bill as late as possible to suppress dissent.
Senator Siewert's speech, seen by AAP, shared stories of multiple Australians who are fearful for when the coronavirus supplement ends.
"I stood in here a year ago and thanked the government for doubling the JobSeeker payment when the pandemic hit, saying I had tears in my eyes when I heard," the West Australian planned to say.
"Well, I had tears in my eyes when I heard about this pathetic increase - tears of distress, anger and despair."
Senator Siewert is dismayed the government tied the increase to more obligations on people looking for jobs.
From July, JobSeekers will be expected to apply for 20 jobs per month.
Thursday is the Senate's last sitting day before 31 March, when the coronavirus supplement ends for welfare payments.
With the clock ticking, the Morrison government voted to push through the welfare bill without debate.
Senators did not get to speak on the bill.
A Greens amendment for the JobSeeker rate to be above the poverty line only had support from crossbench senators Jacqui Lambie, Stirling Griff and Rex Patrick.
Labor argues it did not try to increase the payment so the bill would pass, and has not settled on a figure it thinks is adequate.
"Constitutionally, only the government can legislate money bills. Futile amendments do nothing but cruelly offer false hope to people who need the new JobSeeker rate the most," the opposition's social services spokeswoman Linda Burney tweeted.
"Labor will not be party to cruel stunts or games of chicken with people's lives."
JobSeeker will increase to about $44 a day for a single person with no children, an amount welfare advocates say is nowhere near enough to prevent Australians from living in poverty.
UnitingCare Australia's national director Claerwen Little has said the charity is preparing for an "avalanche" of people who will no longer be able to pay their bills, afford groceries or accommodation.
The Greens wanted to increase the payment to $80 a day.
The coronavirus supplement was initially $550 per fortnight and is now $150.
Social Services Minister Anne Ruston said the increase would cost $9 billion over four years.
"These changes strike the right balance between support for people while they look for a job and incentives to work while ensuring the sustainability of our social security safety net."