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The incident happened on the evening of Friday, April 29 while the man was on duty in Dee St.
Police say the back window of the van the man was in was smashed, and the offender reached inside with a waratah (heavy steel post) and struck him in the lower back.
The victim was treated in hospital as the sharp end of the post pierced his skin, causing soft-tissue injuries.
“This was an unprovoked, serious assault using a weapon on a member of the public doing their job helping keep our roads safe, Detective Senior Sergeant Mark McCloy said.
“We’re hoping members of the community can help us with information which will track down the person responsible, as police want to hold them to account.”
Police want people to watch Ten 7 Aotearoa tonight at 7.30pm on TV2.
"And if you have any information to share, please get in touch."
Contact police at Invercargill station on (03) 211 0400, or via 105 (phone) or online (105.police.govt.nz).
People can also call anonymously using the Crimestoppers line 0800 555 111.