"As expected, the removal of the one-off $1000 kick-start has reduced the incentive to register children in the scheme."
He said the value over a lifetime of the employer contributions and the annual $521 tax credits still provided strong incentives for working New Zealanders to join.
Over all ages, the number of people in KiwiSaver increased by 11,000 in June to 2,530,919.
Finance Minister Bill English said there was an increase of 11,656 in the number of working age New Zealanders, a group the Government had not expected to be impacted by the removal of the payment.
However, he said it was only one month's data and the impact would be clearer over time.
The numbers joining were lower than in May when 14,657 joined.
However, he said May's figures were high because KiwiSaver providers had cleared their backlogs of applications on Budget Day to ensure those already in the system secured the kickstart payment.
Mr Robertson said new enrolments were 29 per cent lower than the average over the previous 12 months and there was also a drop-off in the number of people who voluntarily joined the scheme, rather than through auto-enrolment -- down by 35 per cent on the average.
Mr Robertson has complained to the Speaker after the Prime Minister said advice given on removing the kickstart was that it would "not make a blind bit of difference" to whether people joined the scheme.
Later Budget documents were released showing Inland Revenue had predicted lower numbers of members among the self-employed and children.
The Speaker has dismissed Mr Robertson's complaint.