If you eat New Zealand lamb you are eating Halal meat, without a doubt.
"In answer to your query, I can confirm that much, but not all of the lamb exported from New Zealand is processed in plants that comply with halal requirements. The New Zealand industry does not attempt to hide this fact, but as New Zealand lamb is often not exported in a ‘retail-ready’ form, the decision to label lamb as halal or non-halal is often beyond our control...
Dan Coup
Trade and Economic Manager
Meat Industry Association, New Zealand".
The fact is Muslims and Jews are allowed to slaughter animals in the Halal and Kosher ways for Muslim and Jewish consumption only in the UK.
For such meat to be made available to the whole community is illegal; halal and kosher slaughter is a concession to UK slaughter laws to recognise the needs of Muslims and Jews
Complain and complain again to your MP and Defra. This is wrong on all levels and is ILLEGAL.