The Family and Childcare Trust (FCT) said the increase in Wales between 2014 and 2015 is far larger than rises in England and Scotland.
It also warned that there is not enough holiday childcare to meet demand.
Welsh councils said they are "working hard to support families in need of childcare, including during the summer holidays".
The FCT surveyed local authorities in Wales and found the average price of public sector holiday childcare, such as schemes run by councils or in schools, had risen by 69.9%.
The average increase between 2014 and 2015 across Wales, England and Scotland was 14%.
Jill Rutter, who carried out the survey, said: "Welsh local authorities have until very recently protected a lot of their public sector childcare.
"In England the cuts started much earlier, back in 2010, now things are really catching up on Welsh local authorities."
The average price of public sector holiday childcare in Wales is £108.31 a week. The average price in the private sector is £117.13 a week.
The Welsh Local Government Association (WLGA) said it is working with the Welsh government and childcare providers from the voluntary and private sectors to "consider the issues that parents face in finding accessible and affordable childcare".
The FCT said 95% of Welsh councils lack sufficient holiday childcare for working parents and not one provides sufficient childcare for disabled children, compared to 87% and 88% of English councils respectively.
A spokesman for the WLGA said: "Local authorities in Wales carry out childcare sufficiency assessments which highlight any gaps in childcare provision.
"There is, however, no legal obligation for councils to provide their own holiday childcare."

Canon Bill Brassell from Gloucester served in the Royal Navy in the 1940s, and battled storms, bombers and U-boats to ferry war supplies to Russia.
He has been awarded the Arctic Star medal for the maritime missions, which Winston Churchill said were the most dangerous of the war.
"I'm very, very grateful to receive this," he told the BBC.
"It's only 70 years I suppose, and at last it's happened. Not just because it's a nice thing for me, but because of all those who served with me and they paid the supreme sacrifice.
"Of all the bits that one served during the war, this was the effort that was made by so many.
"I take it [medal] very much in memory of all those who didn't come back, all my shipmates, and that's something that I will very much treasure".
The medal presentation took place at the headquarters of Gloucester-based A Company 6 Rifles.
In 2005, an Arctic badge was awarded to the veterans of the convoys but they felt that it did not go far enough.
They lobbied the government for a specific medal to recognise the service of those involved in the mission.
The Arctic Star medal was created this year.
"This is a medal that we've introduced to honour those people who served on the Arctic convoys in World War II," said Conservative MP Mark Francois, the minister of state for defence personnel, welfare and veterans.
"I'm privileged to present this to Canon Brassell."
The Arctic convoys brought crucial supplies and weapons to Russia, to help fight off Hitler's armies.
The icy voyages claimed over 3,000 lives and Winston Churchill described them as the worst journey in the world.
The supply ships were under constant threat of attack by German U-boats and aircraft and also had to negotiate severe cold, storms, and ice floes.
The voyages began in 1941 and by May 1945, the Arctic route had claimed 104 merchant ships and 16 military vessels.
The cost of council-run school holiday clubs has increased by nearly 70% this year, according to a charity.
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An 89-year-old Gloucestershire man has been honoured for his efforts in the World War II Arctic convoys.