There were only about 500 centres, many being medical clinics and pharmacies, that had been accredited for paediatric vaccines, compared to the roughly 1000 available to administer adults with the jab, he said.
"So there's a bit of discrepancy there. We expect the numbers to build over the next few weeks," Betty said.
He urged parents and caregivers to ring ahead or use the Ministry of Health'sBook My Vaccine
To become accredited to administer Covid vaccines to children, meant health professionals had to be up to speed with how much and how the vaccine should be given to children and get ministry sign-off.
"This was always going to be problematic over Christmas and New Year in terms of reduced capacity [of health professionals] in the system," Betty said.
To the ministry's credit, Betty said the 500 paediatric vaccine sites that were available, were evenly distributed across the country.
"But again, unless you knew where they were, you wouldn't be able to pick it," he said.
Auckland Covid vaccination programme clinical lead Dr Anthony Jordan said some sites were yet to become available online and that was partly to do with when their staff were back on board after the summer break.
"Once all of our paediatric sites are open, there'll be a lot more options and availability, especially through the booking system," Jordan said.
Pharmacy Guild of New Zealand Chief Executive Andrew Gaudin said the rollout of the paediatric vaccinations was being managed by individual DHBs across the country, so the Guild didn't have an overview of the national situation.
"However, we have not been advised of any systemic issues with the supply and distribution of vaccines for the paediatric vaccination rollout.
"As far as we are aware, the rollout is underway and the number of pharmacies that are delivering the paediatric vaccine will continue to grow in the coming days," Gaudin said.
Earlier this month, a ministry spokesperson said deliveries of the child vaccine doses were scheduled to commence from January 10, to ensure all sites nationally had child doses available in readiness for today's start date.
Betty said as expected there had been a slight issue at a few places but overall the distribution pathways was very well formulated now compared to six months ago.
"I wouldn't expect a lot of problems with this to be honest," Betty said.