A death notice yesterday said Dr Senior "died in pursuit of his dream''.
His partner was at Muriwai when the accident happened but it was not known if she saw the impact.
Several other people were also there.
"It was extremely disturbing for everyone who was out there,'' Mr Nuttall said.
Engineers were helping police, but it was difficult to know which other agencies should be involved, he said.
"If it was something of commercial interest Maritime New Zealand would have an interest. If it was a car accident we would be able to do our usual investigations -- that's our bread and butter -- but it was a personally built piece of machinery for personal use.
"It's something out of the norm for us.''
Inquiries were continuing.
He urged anyone who saw the accident to contact police.
- NZPA