Authorities in Waikato have upped efforts to find the three children of missing Marokopa man Tom Phillips.

An $80,000 reward has been posted for anyone with information, with a warrant out for Phillips' arrest.

Acting Detective Inspector Andrew Saunders suspects other people are assisting Phillips - and hopes the reward and immunity will draw them out.

"That's one of the reasons why we put this reward up for a limited period of time, to encourage those people to come forward."

LIASTEN ABOVE

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• Still armed and dangerous: Tom Phillips hiding in or near Marokopa, cops say