To investigate the quality of diagnostic and triage advice provided by free website and mobile application symptom checkers (SCs) accessible in Australia. 36 SCs providing medical diagnosis or triage advice were tested with 48 medical condition vignettes (1170 diagnosis vignette tests, 688 triage vignette tests). Correct diagnosis advice (provided in first, the top three or top ten diagnosis results); correct triage advice (appropriate triage category recommended). The 27 diagnostic SCs listed the correct diagnosis first in 421 of 1170 SC vignette tests (36%; 95% CI, 31-42%), among the top three results in 606 tests (52%; 95% CI, 47-59%), and among the top ten results in 681 tests (58%; 95% CI, 53-65%). SCs using artificial intelligence algorithms listed the correct diagnosis first in 46% of tests (95% CI, 40-57%), compared with 32% (95% CI, 26-38%) for other SCs. The mean rate of first correct results for individual SCs ranged between 12% and 61%. The 19 triage SCs provided correct advice for 338 of 688 vignette tests (49%; 95% CI, 44-54%). Appropriate triage advice was more frequent for emergency care (63%; 95% CI, 52-71%) and urgent care vignette tests (56%; 95% CI, 52-75%) than for non-urgent care (30%; 95% CI, 11-39%) and self-care tests (40%; 95% CI, 26-49%). The quality of diagnostic advice varied between SCs, and triage advice was generally risk-averse, often recommending more urgent care than appropriate.