July 28 (Reuters) - Westpac Banking Corp said it gave updated information on more than 500,000 threshold transactions to Australia's financial crime regulator that it had not reported or were incomplete amid allegations it enabled millions of illegal payments.

But Australia's second-biggest lender said on Tuesday that not all of the transactions of A$10,000 or more it provided the regulator may have breached anti-money laundering laws.

Westpac is facing allegations that it enabled millions of illegal payments, including between known child sex offenders in a scandal that has resulted in a major management overhaul and the lender's setting aside A$900 million ($643.05 million) for a possible penalty.

After looking deeper into its systems and in response to a notice from the regulator, AUSTRAC, Westpac said it provided about 175,000 transactions it had not reported and about 365,000 that it had but for which it may not have had complete or accurate information.

"A significant proportion of the potential reporting issues relate to a range of complex scenarios where the legislation requires Westpac to exercise judgement on how multiple transactions may be aggregated and whether a threshold transaction has actually occurred," the bank said in a statement.

It said it was still working with AUSTRAC and that the number of threshold transaction reports may change. ($1 = 1.3996 Australian dollars) (Reporting by Nikhil Kurian Nainan in Bengaluru; Editing by Sandra Maler and Leslie Adler)