The public can now get their hands on a series of four souvenir medallions celebrating the Istana's rich diversity of flora and fauna after they were launched yesterday.

Two of the medallions - available only in a special four-in-one set - depict the oriental pied hornbill and barn owl among tropical fruits and spices, The Singapore Mint said.

Only 100 of the special medallion sets were available at the National Day Open House yesterday.

The other two - which feature the Istana Vanda Miss Joaquim and the Istana swans - can be bought individually, with 300 pieces of each design going on sale yesterday.

All medallions come with the official Istana logo on the reverse side, and are accompanied by nutmeg-inspired motifs that pay tribute to the Istana's origins as a nutmeg plantation.

A special Nets Flashpay card will also be issued.

The four-in-one set is priced at $438, while the Istana Vanda Miss Joaquim and Istana swans medallions cost $110 each.

The series - the second of its kind this year marking the Istana's 150th anniversary - was launched by President Halimah Yacob.

Part of the proceeds will be donated to the President's Challenge, the annual community outreach and fund-raising campaign.

Those with any inquiries can call The Singapore Mint on these phone numbers: 6566-2626, 6222-2486, 6336-2878 or 6634-2009.

To mark the Istana's 150th anniversary, President Halimah yesterday also launched a new series of three books titled 150 Best Kept Secrets Of The Istana, with each containing 50 little-known facts.

Illustrations in the front and back spreads of the books were created by beneficiaries from volunteer welfare organisation Club Rainbow.

The books will be available for purchase from major local bookstores at a retail price of $15 from today, with net proceeds also going to the President's Challenge.

Speaking to the media yesterday, President Halimah said that the books are written in short bite-sized prose to make them more readable.