The Transport Accident Investigation Commission (TAIC) says if it had more money it could use more technical expertise and staff in its investigations.
The widow of a helicopter crash victim near Wanaka in 2014 has complained about how long TAIC is taking to investigate the accident.
Documents obtained by Radio New Zealand under the Official Information Act reveal TAIC has struggled with its level of funding.
Chief executive Lois Hutchinson said more money would help investigations.
"It's not something that we can necessarily speed up. The difference is that we would have more expertise readily available and, if you like, more boots on the ground," she said.
Ms Hutchinson said investigations could not be completed quickly but the commission met international guidelines.
Adelle Box, whose husband Jerome died when a Squirrel helicopter carrying seven people crashed near Wanaka in August 2014, still questions why that investigation is taking so long.
But Mrs Box is particularly critical of what she says is the commission's failure to communicate with her over its investigation.
"I understand that they can't specifically answer the questions. All I was looking for was some sort of guarantee that they are being very thorough and I would really, really, really like to see some indication of actually when the report will be ready...I don't want to see this done twice. I want some confirmation and some confidence that they're doing the right thing," she said.
Ms Hutchinson said the commission had not communicated with Mrs Box as often as she wanted or needed.
"I think we for ourselves are mindful of the challenging, the difficulty for families who want answers, want to know, need to know and to get closure. I have the utmost sympathy for them in that position. We do accept that we can do better."
Ms Hutchinson said the commission was setting up direct contacts with victims' families so it could better communicate with them.