The Police watchdog has found serious deficiencies in an investigation into a fatal Northland crash

Two people were killed in the April 2018 crash and a tourist was initially charged with careless driving causing injury or death.

A few hours later that was upgraded to aggravated careless driving -- a move the Independent Police Conduct Authority says was not supported by enough evidence.

It also found statements from survivors were not sufficiently detailed, or fact-checked appropriately.

The tourist later argued there wasn't enough evidence to go to trial, a judge agreed and dismissed his charges.

Police say they accepted the findings, and have made changes.