Australia's $300 million plan to impose GST on imported online purchases from July 1 is at risk of collapse with the world's largest online marketplaces - eBay, Alibaba, Etsy and Amazon - considering blocking Australian customers when the new regime begins.

When Parliament resumes after the May budget, Labor and crucial Senate crossbencher Nick Xenophon will push to delay by at least 12 months Australia's world-first attempt to tax online marketplaces as if they were retailers after warnings from e-commerce giants, freight companies, consumer groups and tax exports that the proposed model is impossible to implement in just two months.