CPA Australia has said that while the government's proposed GST changes will address companies such as Amazon, there remains a plethora of smaller retailers that will escape examination.
"Even assuming we can get large retailers such as Amazon to apply the GST to Australian purchases, there is a significant number of small online retailers around the world that have to be dealt with," CPA Australia chief executive Alex Malley said.
"So while you may be able to address the global elephants, like Amazon, the risk is that we get overtaken by the millions of mice following along."
Malley said Australia is out of touch with other nations in having a AU$1,000 GST threshold for online purchases, and it is putting Australian companies at a competitive disadvantage.
"The problem that is holding us back from lowering, or even removing, the threshold is the massive administrative costs it would create," he said. "It's dealing with this administrative burden and designing cost-effective collection processes which is occupying the minds of policy makers.
"Part of the discussion will involve the big international suppliers having to register for GST as though they were an Australian resident, a process that could be made to work with the assistance of the G20."
In April, Treasury deputy secretary Rob Heferen told a Senate inquiry that compliance costs would be very low if international providers of imported goods had to register and pay the GST.
On Monday, federal Treasurer Joe Hockey said Australia would be asking online retailers directly to collect GST.
"What we've identified is a way ... to be able to impose the GST on the supplier overseas -- other countries have passed those laws as well -- so now we can go to the Amazons, we can go to the various retailers overseas and say, 'You have to apply the GST to goods that you are selling into the Australian market', and they will do so," he said.
"That's how we are going to collect it."
Hockey also said the threshold may be lowered to zero, which is under discussion between himself and the state treasurers.
Communications Minister Malcolm Turnbull backed the direct approach, saying large retailers already have the systems to cope with such a change, and the federal government should be collecting GST from online purchases.
"We've seen this with the online platforms where they've been asked to collect differentiated tax rates," he said. "Go onto Amazon and order something, put in an address in New York and you'll get one set of taxes, change it to an address to another state and you'll see another set of taxes."