JAKARTA (REUTERS) - Indonesia, in a bid to boost participation in its pivotal tax amnesty, has told taxpayers wanting to incur the smallest penalties that they can pay now and supply needed paperwork later.

The move comes one week before the Sept 30 end of the amnesty's first phase, during which the lowest penalties for registering previously-unreported assets apply.

Penalty rates of two per cent for onshore assets and those repatriated from overseas rise after September.

Since the nine-month amnesty began in July, participants have had to file all documentation at the time claims were submitted. That led some businesses to complain that the administrative requirements were complicated.

Now "amnesty participants who want to get the lowest rates must join before Sept 30, but we have agreed to let them submit the paperwork later," Finance Ministry spokesman Luky Alfirman said on Friday (Sept 23).

People who sign up and pay estimated penalties by Sept 30 will be allowed to later revise their asset declaration, he said, but any additional asset declared after Sept 30 will carry the higher penalty tax rate for post-September phases.

Mr Rosan Roeslani, chairman of the Indonesian Chamber of Commerce and Industry, welcomed the extra time for paperwork, saying documents "can take a long time to collect".

The chamber had proposed extending beyond Sept 30 the deadline for the most favourable terms, which the government rejected.

After a slow start, the programme has picked up pace with 100,000 Indonesians taking part and declaring more than US$110 billion (S$149.2 billion) of assets, the finance ministry said on Thursday.

The government is banking on the amnesty to bring in 165 trillion rupiah (S$17 billion) in 2016 to help keep the budget deficit from breaching a legal limit of three per cent of gross domestic product.

As of Thursday, the programme had generated 35.1 trillion rupiah in revenue, or 21 per cent of the government target.