"The kids and their parents who arrived after 9am never even knew it had happened."
Tag Off volunteer Walter Te Huia, who is also a police youth development worker, had a warning for the culprits.
"Graffiti is a crime and when we catch those responsible, they will be held to account," he said.
The 18-week upgrade of the netball courts cost the council about $650,000.
Ms Brophy said the upgrade was essential because the 40-year-old courts were dangerous and the Netball Centre's 1200 players deserved a better home venue: "Three of the courts had ditches and trenches, which meant we couldn't use them, and the run-off areas were too short and didn't meet New Zealand Netball standards for tournament play."
The upgrade also included lighting for four of the eight courts.
Mr Carter said the Bay of Islands Netball Centre and its patron, Jim Lawrence, had lobbied council to upgrade the courts. The project had taken a long time but the result was well worth the wait.