Here is the basic rule. It does not matter if someone is entering the US under the visa waiver program which permits stays of up to 90 days, or if the person has a B2 visitor's visa which permits stays of up to six months.  The visitor must spend more time in their home country than they spend in the US.  Otherwise, they are considered as living in the US with visitor visas which is not allowed.  Since this was her third trip in nine months, she probably was spending more time here than in Australia and it is the likely reason she was denied entry.