In a critical report, the Senedd’s public accounts and administration committee said the decision was taken with a “lack of thorough due diligence”.
Plans were “not sufficiently robust and had not been communicated effectively to the community”, it said.
It added a failure to keep an adequate record of meetings with Green Man officials meant the Senedd was “unable to fully scrutinise and evaluate decisions taken by the Welsh government”.
Decision-makers in the Welsh government were also not provided information about the purchase in a “timely manner”, the report said.
The committee said the “haste” that the government bought the site in may have also inhibited its ability to identify risks around the presence of wildlife “that would affect its proposals for the site, and potentially, its value”.
“This is particularly notable as the site has now been valued at £3.75m, meaning that the Welsh government’s asset has lost half a million pounds in value,” the report said.
Committee chairman Mark Isherwood said: “The arrival of the ospreys on the site was unexpected and it’s acknowledged that the Welsh government has responded positively to this development to preserve their habitat, albeit there was evidence of other protected species being present at the site at the time of purchase.
“However, the future of the site now appears to be very uncertain, with the most recent valuation showing that the asset has decreased in value by £0.5m compared to the purchase price. This is highly regrettable.”