My understanding of how duty/taxes are collected on imports in the UK is the carrier assigned to deliver the goods in the UK holds the goods until the duty and or taxes have been paid which is determined based on the customs declaration attached to the parcel. In other words, the carrier is collecting the taxes on behalf of the government.
The purchaser must pay the duty/taxes before the carrier forwards the goods, so, no payment, no goods.
This is why we quite often see sellers on ebay or Aliexpress offering to understate the value of goods to lesson the tax/duty the purchaser pays when the goods reach the destination country. This is the only feasible way the government is going to collect GST on overseas purchases that I can see. i.e a mixture of carriers and customs collecting duty and or taxes. It can't be based on customs collecting duty and taxes alone, otherwise the country will come to a standstill.
Some carriers are authorised to perform customs clearance duties in the UK, not sure if it's done here in Oz, but I have an inkling that some are.