Was a bit mangled but it seems party was on Saturday, infected person developed symptoms on Sunday, positive test results returned on Monday, contact tracing started, positive results returned on Tuesday for a number of party participants including preschool child.
Dr Chant said there was a period where they would not have been infectious (Sunday) but all those individuals would have unknowingly had the infection on Monday. So it seems the Monday she referred to was the day when they could have spread the infection not the party itself. Sydney Morning Herald have reported party as Saturday and NSW Health reported Infected child's attendance at child care on the Monday.
" A total of 10 people who attended the party have now tested positive for COVID-19, including a two-year-old child, who attended Little Zak’s childcare in Narellan Vale on 21 June."