Nearly 10,000 alerts were sent to users of the NHS Covid-19 app in Wales in the latest recorded week, the highest number since mid-July.
A total of 9,753 pings were transmitted in the week to August 18, informing people they had been close to someone who had tested positive for coronavirus.
This is up 63% on the previous week, Government figures show.
Meanwhile, in England, the number fell slightly to 237,764 alerts, dropping 7% on the previous week and the lowest since the week to June 23.
Since August 7, fully-vaccinated contacts in Wales no longer have to self-isolate if identified as a close contact.
The latest data is the first full week to cover similar rule changes for England, which came into effect from August 16.
Venue check-ins have also fallen below one million between the two nations, with 32,381 QR code app scans carried out in Wales and 958,592 in England.