NSW recorded 15 new local cases of COVID-19 in the 24 hours to 8pm on Thursday, while another case was uncovered in a person who had since travelled to Queensland.
Another 10 people were confirmed virus-positive after 8pm on Thursday.
Two cases on the northern beaches had been confirmed in the 24 hours to 8pm on Wednesday.
The peninsula has entered a quasi-lockdown, with NSW Health asking residents to stay home and not leave the area for the next three days. Aged care facilities in the area have been shut to visitors.
NSW Premier Gladys Berejiklian implored northern beaches residents to only leave home for essential reasons over the next few days.
Health authorities on Thursday night used genomic sequencing to track the outbreak of the cluster to an international case.
"The harder all of us work together, the better Christmas we'll have," Ms Berejiklian told reporters on Friday.
NSW Chief Health Officer Dr Kerry Chant confirmed to reporters on Friday that a major COVID-19 seeding event had occurred at Avalon RSL on December 11, followed by a similarly significant seeding event at Avalon Bowlo on December 13.
Of the 28-strong cluster, eight people in the cluster were in attendance at Avalon RSL, 16 were at Avalon Bowlo and two were at both. The final two cases were also connected to those venues.
Ms Berejiklian said that from Tuesday all international air crew arriving in Sydney would be held at two quarantine hotels until their flight back out of the country, typically within 72 hours.
Ms Berejiklian admitted airlines were currently in charge of ensuring air crew quarantine compliance, but NSW Police would from Tuesday take over.
"The issue is not the guidelines we have in place, it is unfortunately a few occasions where people breached the guidelines or actually chose not to self-isolate when they should have," Ms Berejiklian said.
"The systems themselves are not been a challenge but it has been people disobeying what is in place."
TRAVEL RESTRICTIONS ON NSW RESIDENTS
* Western Australia - Anyone who enters WA from NSW must self-isolate for 14 days. Those already in WA must self-quarantine for 14 days, starting from the date they were last on the northern beaches.
* South Australia - Anyone in SA who has recently arrived from NSW should follow NSW Health advice by self-isolating for 14 days and get tested if they have visited any of the venues on NSW's health alert list.
* Northern Territory - Anyone who enters NT from Sydney's Northern Beaches Council area will need to undergo 14 days of supervised quarantine.
* Queensland - Anyone who enters Queensland and has been on Sydney's northern beaches since December 11 must get tested and self-isolate for 14 days since their last visit to the region. From Saturday, all people who have been on the northern beaches and want to enter Queensland will be forced into hotel quarantine.
* ACT - Anyone who enters the ACT and has been on Sydney's northern beaches since December 11 must get tested and self-isolate.
* Victoria - Anyone who enters Victoria from Sydney's northern beaches must get tested and self-isolate for 14 days.
* Tasmania - Anyone who has visited the Northern Beaches Council area since December 11, as well as the Kirribilli Club, Penrith RSL and Gannons Park in Peakhurst at various times between December 11 and December 14, is barred from entering Tasmania.