[2][5][6] Hilliam researched and documented a large number of shipwrecks in the Northland Region, including the numerous wrecks at the Kaipara Harbour.

[8] In 1982, Hilliam reported seeing the wreck of a Spanish ship while flying over Baylys Beach, but the "swirling sands quickly covered the find over again".

[9] In the same year Hilliam identified a wooden ship exposed in the shallow waters of Midge Bay, north of the Kaipara Harbour entrance.

[15][16] In 1988, New Zealand archaeologist Michael Taylor submitted 14 pages of notes about the structures to the National Archives with the condition that access to the records be restricted for 75 years.

In reality, the request was made out of respect to the privacy of the local iwi due to the inclusion of "personal and family information" pertaining to their cultural group.

An unnamed, "Oxford-educated" researcher examined these claims, based on Hilliam's belief that a Spanish ship visited in the 16th century and sank near Aranga on Northland's west coast.

Robert Langdon's book The Lost Caravel suggests it sunk at the Tuamotu Archipelago due to speculation about Polynesians with European features and the use of Spanish words there.

[9] In 2012, Hilliam co-authored To the Ends of the Earth, which controversially argued that the Māori demigod Maui was not Polynesian but an ancient Egyptian navigator.

Hilliam used an unnamed "expert in computer-imaged facial reconstruction"[28] to create images purporting to show what the pre-Māori settlers looked like.