The use of "trespasses"

I don't know about "legitimate", but it's not unique to this article. A Google search for "trespassed him" finds examples such as:

Deputies inside the courtroom trespassed him from the courtroom and notified Perry, court spokeswoman Karen Levey said. —Orlando [Florida, U.S.A.] Sentinel, June 2012

A drunken man was talking to himself inside a coffee shop and acting rude to customers, according to a report. Police trespassed him from the property for a year. —Sarasota [Florida, U.S.A.] Observer, March 2012

He refused to leave when asked and police trespassed him from the store. —Savage [Minnesota, U.S.A.] Pacer, June 2012

I would have fired that guy right there on the spot , and trespassed him right off the property- […] —some Netizen posting under the name "Old Gobbler" with a signature suggesting that he lives in or near Osceola, Florida, U.S.A.; June 2011

That customer broke up the fight, after which the manager fired the employee and trespassed him from the eatery. —Columbia [South Carolina, U.S.A.] Star, May 2012

In these contexts, it seems to mean either "to escort out of" or "to bar from", with the added implication that failure to comply would constitute trespassing.

Notice that the cited article is from New Zealand where this usage is apparently common. There, to "trespass" someone is to serve them a notice that you want them to leave your property ... if they remain, or return, then they will have committed criminal trespass (https://www.police.govt.nz/advice-services/personal-and-community-safety/trespass-notices).

Note that this usage is similar to saying that someone has been banned or "86ed"--meaning (following Wikipedia or MW) to refuse to serve, to get rid of, or to throw out.

So the headline 'Criminal trespasses police officers' could be read as

Criminal bans police officers

and

The entire police force has in effect been banned from a Wellington property.

As for your final question, whether it's "legitimate" depends on whether you're a descriptivist or a prescriptivist, and if the latter, what your standard is. One could argue either that the lack of entries in dictionaries makes it illegitimate, or that repeated occurrences of this usage, found via web search, makes this a new usage that will eventually be stamped with legitimacy by lexicographers adding it to dictionaries.

I wrote a column about just this question over at Visual Thesaurus, with a tip of the hat to ruakh's answer. As I write there, the earliest attestation I found is from 1990; also, this usage seems to be popular in New Zealand, the source of the OP's newspaper headline.

"Trespassed" is a term used in New Zealand in reference to delivering a "Trespass Notice" to someone. A trespass notice does not mean that the person has actually been trespassing on the property, but instead is a notice that if they do not leave the property and/or if they return to the property in the future then they will be trespassing on that future occasion.

Once the trespass notice has been delivered to the person (verbally or in writing, either by the property owner/representative or the police) they are deemed to have been "trespassed" from that property under whatever the terms of the notice are.