It always makes me furious, when people, particularly politicians, resort to cherry picking in order to back their far fetched claims.
But what's that German news article linked below about?
In short, Tony Abbot claims that, since Germany does not have data retention, while Great Britain does have it, clearance rates for child abuse crimes are much higher in Great Britain than in Germany.
Well, numbers say something different...
Luckily, Germany publishes the clearance rates sorted by crime here (there's also an English PDF):http://www.bka.de/DE/Publikationen/PolizeilicheKriminalstatistik/pks__node.html
For Great Britain I didn't find any prepared numbers for the clearance rate of (sexual) child abuse crimes, but one can calculate the outcome rates using the detailed raw data available here (that's actually quite depressing, as alone the sheer size of the tables illustrates how many crimes are being committed):https://www.gov.uk/government/statistics/police-recorded-crime-open-data-tables
A condensed report of outcome rates for England/Wales is available here, but lacks a separate listing for (sexual) child abuse:https://www.gov.uk/government/statistics/crime-outcomes-in-england-and-wales-2013-to-2014
It turns out that the clearance rate for sexual child abuse is much higher in Germany (around 84% in 2012/2013) than the outcome rate for the same crime is in Great Britain (around 30% in the same years).
While this doesn't proof anything (again I've taken just one example - I only compared Germany and Great Britain for two years, completely neglecting all other countries and time frames), it certainly hints that data retention might not be the most important factor when it comes to crime clearance rates.
Disclaimer: I'm not absolutely sure that the German "Aufklärungsrate" can be directly compared to the British "outcome rate". Please correct me if I'm wrong.