The federal government is being urged to delay charging GST on items bought from overseas worth $1000 or less.
The measure, which passed parliament's lower house on Wednesday, is set to start on July 1.
But Labor, while giving in-principle support, wants to postpone it by a year, as recommended by a Senate committee.
It also wants the Productivity Commission to work out the best way to implement the measure and report back by the end of 2017.
Shadow treasurer Chris Bowen accused the coalition of botching the change, which was announced in the 2016 budget and introduced to parliament in February.
"This government has given Australia's businesses and consumers a fortnight to change their models and implement the new regime," he told MPs.
"This is a very significant issue."
Labor would move amendments in the Senate and call on the crossbench to support its "sensible" proposal.
Assistant Minister to the Treasurer Michael Sukkar said the existing arrangements were unfair to local businesses and needed to change.
"Another review will not deliver new information and will only delay reform by at least another two years," he told parliament.
"We must act now."
The debate came as the Australian Retailers Association launched a campaign calling on politicians to pass the legislation.
The group has been lobbying the government to reduce the threshold from $1000 to zero to help provide a "level playing field" for Australian retailers.
"Australian retailers are continually having to pay GST while their foreign counterparts do not, and local online and bricks-and-mortar retailers are sick of the constant delays in implementing this tax equality issue," executive director Russell Zimmerman said in a statement.
"We do not want to take away the right for consumers to purchase from overseas. However, we do need to provide a level playing for Australian retailers."
