The Commerce Commission says an investigation into ING's two frozen funds will not be completed for at least another six months.
ING's diversified yield fund and regular income fund were originally worth more than $500 million but they were frozen last year after their value crumbled.
Investors have to decide by 13 July whether to accept or reject a settlement offer of 60 cents by ING. If investors accept the offer, they waive the right to legal action.
The commission is investigating the promotion and sale of the funds to see if there were any breaches of the Fair Trading Act.
On Wednesday, it advised the investigation will not be completed for another six months. It recommends investors take their own legal and financial advice before making a decision.
Fair trading manager Graham Gill says the process could take another six months to three years to complete if litigation is deemed appropriate.
Financial adviser Paul Markham says ING investors face a quandary as the outcome of the commission's investigation will not be known before they decide on the settlement offer.
Mr Markham says the situation is forcing investors to make a difficult decision and most will probably accept ING's offer for the certainty it offers.