(anyone else cringe at holding drinking water in a thing called a cist-urn? Cause I do)
Depends on a lot of variables. A cistern collecting rainwater from a surface is easily contaminated. one collecting rainwater from the shallow water table is susceptible to ground contamination. When there is a slow rate of replacement the large diameter of a cistern can collect usable amounts of water between uses. If it is well sealed with clay to prevent surface rain entering and the water enters through a gravel and sand layer in the bottom from clean
land I would drink it. Had such a well in Maine; one the edge of woodland, deep clay with water seams 6 to 8 feet deep, was dug 16 feet deep with back hoe, bottom layer of gravel and sand, cement culverts were stacked with gravel around them to the 6 foot level then packed with clay to the surface. Remained full to the surface most of the year with very good water.
Hopefully the design there is well done and your maintenance plan is good then when it is settled have the water tested.