Barbie Ashley, her goddaughter, tells TMZ ... Esta died unexpectedly on Thursday in her North Hollywood home ... medical personnel believe the South African actress had been dead for about a day before she was found.

The cause of her death is under investigation, her rep, Lisa Rodrigo tells us ... adding there is no other information at this time.

Esta was crowned Miss Teen South Africa in 1991 ... and from there landed a number of acting roles ... including her most well-known ... Gillian Andrassy, a trouble-making princess, who was killed off in 2001.

She is survived by her ex-husband and goddaughter.