Can I challenge a will on the grounds the will was forged or a fraud was perpetrated?

Yes, however the onus of proving fraud lies with the person who alleges it. You need to convince a judge with your evidence. To some extent you need to produce evidence just like you would with a claim for any wrong doing. The onus is on you to prove your case with evidence. A Judge said in one case:

“Fraud, sufficient to result in the invalidation of a testamentary instrument, is concerned with misleading or deceptive conduct. With fraud, there is no overpowering of the volition, no coercion. Whereas undue influence coerces a testatrix, fraud misleads her. Willfully false statements, or the suppression of material facts, intended, either, to gain for oneself benefits under a will, or to prevent benefits being received by a natural object of the testator’s bounty.”

In another case the Judge distinguished between undue influence and fraud. He observed that there had been a plea of undue influence in the case but no clear plea of fraud. He also said “If you intend to charge the plaintiff with having obtained the execution of this will by instilling into the mind of the deceased false and delusive notions respecting the conduct of the defendant, this is tantamount to a charge of fraud, and you should have placed on the record a plea charging her with having obtained it by fraud…”

Therefore, in order to succeed with an allegation of fraud, it must be firmly established that the fraudulent conduct had a direct effect on the making of the will and, that the fraudulent conduct was perpetrated by the beneficiary for the sole purpose of receiving a benefit under the will. If issues involving both fraud and undue influence arise on the facts then each ground for contesting the will must be pleaded separately and properly.