More than 2000 vials of a banned steroid have been found by Customs officials inside a parcel sent to Sydney from Thailand, it has been revealed.

Officers at Australia Post's gateway facility in Clyde allegedly found 2378 ampoules of Sustanon 250, Stanozol and Nandrolone in plastic bags after examining the parcel, which was meant to contain glass figures.

The steroids are classed as performance-enhancing and cannot be imported without a permit from the Therapeutic Goods Administration.

A search warrant was executed on a premises in south-western Sydney last week, national investigations manager Richard Janezcko said last night.

But he could not confirm further details, saying only evidentiary material was allegedly seized as part of a major investigation into alleged smuggling attempts.