Deleting texts does not destroy them.

"Phone companies were required to hold SMS information for up to three weeks and police were able to access that information with a search warrant."
Senior Sergeant Mike Fulcher(2007)

Vodafone is no longer storing texts, and Telecom is about to follow suit (Reported in NZ Herald, July 28, 2007)
Police were able to obtain copies of text messages but a decision by the two big mobile phone companies to stop storing text messages is costing the police an investigatory tool.
The change, caused by the companies' introduction of new equipment, raised debate among police and lawyers about whether a law change should be sought to require the information to be kept. This was seen as not practical, because:

"It [text content] was a source so we will just have to rely on other means," Mr Peoples said.
If police considered an investigation was serious enough, they could apply for an interception warrant which would enable voice and text calls to be captured as they happened.
And, Police can still obtain records of text calls, but not the text.