Interest & Fees

Overview

Interest and fees are the primary ways banks generate revenue from accounts and products. Understanding how they work is important for product configuration, posting logic, and customer disputes.

Interest — The Basics

Interest is the cost of borrowing money, or the reward for saving it:

From Customer's View	From Bank's View
Saves money → earns interest	Pays interest on deposits (liability)
Borrows money → pays interest	Earns interest on loans (asset)

Interest Rate Types

Type	Description	Example
Fixed Rate	Rate locked for a term	5.00% p.a. for 12 months
Variable Rate	Can change at bank's discretion (follows RBA cash rate)	RBA rate + 2.50%
Introductory Rate	Promotional rate for new customers (limited period)	5.50% for 4 months, then 3.00%
Tiered Rate	Different rates for different balance bands	0– $10K: 2%,$ 10K+: 4%
Bonus Rate	Extra interest if conditions met (e.g., deposit each month)	Base 2% + Bonus 3% = 5%

How Interest is Calculated

Simple Interest (Daily Accrual)

Most bank accounts use daily accrual — interest accumulates every day based on the closing balance:

Daily Interest = (Annual Rate / 365) × Balance

Example:
  Balance:      $10,000
  Rate:         5.00% p.a.
  Daily rate:   5.00 / 365 = 0.013699%
  Daily amount: $10,000 × 0.013699% = $1.37

Monthly:  ~$41 (varies by days in month)
Yearly:   ~$500

Compound Interest (Interest on Interest)

Formula: A = P × (1 + r/n)^(n×t)

Where:
  P = Principal ($10,000)
  r = Annual rate (0.05)
  n = Compounding frequency per year (12 for monthly)
  t = Years (1)

A = $10,000 × (1 + 0.05/12)^12 = $10,511.62
Interest earned = $511.62  (vs $500 simple)

Banks use compound interest for loans — it means customers pay more over time.

Interest in Banking Systems

Accrual vs Capitalisation

ACCRUAL:        Interest earned but NOT yet paid
                Recorded as a liability (owed to customer) or receivable (owed by customer)

CAPITALISATION: Accrued interest is added to the principal balance
                (For savings: credited to account)
                (For loans: added to outstanding balance if not paid)

Payment Frequency

Product	Typical Interest Payment
Savings account	Monthly (end of month)
Term deposit	Maturity / monthly / annually
Home loan	Monthly (deducted from payment)
Credit card	Daily accrual; billed monthly
Overdraft	Daily; debited monthly

Common Bank Fees

Account Fees

Fee	Description	Typical Amount
Monthly account fee	Charged for maintaining account	$5-$ 15/month
Excess transaction fee	Fee per transaction over monthly limit	$0.50-$ 2 each
Paper statement fee	Fee for receiving paper statements	$2-$ 5/month
Account closure fee	Sometimes charged on term deposits if broken early	$0-$ 50

Payment Fees

Fee	Description	Typical Amount
International transfer fee	SWIFT outbound payment	$15-$ 30 per payment
Currency conversion margin	Spread on FX rate	1%–3%
BPAY fee	Usually nil (absorbed by bank)	$0 (retail)
Real-time payment fee	NPP — usually nil for consumers	$0

Penalty / Event Fees

Fee	Description
Dishonour fee	Direct debit or cheque bounced due to insufficient funds — $10-$ 15
Overdrawn fee	Account went negative without approved overdraft — $10-$ 20
Late payment fee	Credit card or loan payment overdue — $20-$ 30
Break fee	Exiting a fixed-rate product early (e.g., fixed home loan) — can be thousands

Fee Posting in the System

When a fee is charged, a debit posting is made to the customer's account:

Fee event trigger (e.g., end of month, dishonour event)
        │
        ▼
Fee calculation engine
  (looks up product config, calculates fee)
        │
        ▼
Post debit to customer account:
  DR  Customer Account     $10.00  (Fee: Dishonour fee)
  CR  Fee Income Account   $10.00  (Bank's P&L)
        │
        ▼
camt.054 notification:
  CdtDbtInd: DBIT
  BkTxCd: PMNT/FEES/SRVC
  Amount: $10.00
  Narrative: "Dishonour fee - DD returned insufficient funds"

Regulation: Responsible Lending & Fee Caps

Banks are subject to regulatory limits on fees:

Regulation	Requirement
National Consumer Credit Protection Act	Responsible lending; fee disclosure
Credit Code (NCCP)	Maximum default fees; comparison rate disclosure
ASIC oversight	Unconscionable fees; deferred sales model
RBA guidance	Surcharging rules (merchant passing on card fees)
Fee-free basics (COBA)	Mutual/community banks offer fee-free basic accounts

Interest on a Home Loan — How It Works

Home Loan: $500,000 at 6.00% p.a., 30 years, monthly payments

Month 1:
  Daily rate:   6.00 / 365 = 0.016438%
  Days in month: 31
  Interest:     $500,000 × 0.016438% × 31 = $2,547.95

  Monthly payment: $2,997.75 (fixed, from amortisation schedule)
  Principal paid:  $2,997.75 − $2,547.95 = $449.80
  New balance:     $500,000 − $449.80 = $499,550.20

Month 2:
  Interest:     $499,550.20 × 0.016438% × 28 = $2,299.49
  (fewer days)

Over 30 years:
  Total paid:   ~$1,079,190 (more than double the loan)
  Total interest: ~$579,190

RBA Cash Rate and Its Effect

The RBA Cash Rate is the benchmark interest rate set by the Reserve Bank of Australia. It flows through to customer rates:

RBA Cash Rate (e.g., 4.35%)
         │
         ▼
Bank Funding Cost (deposits, wholesale)
         │
         ▼
Retail Lending Rates (mortgages, personal loans)
  = Cash rate + Bank's margin

Retail Deposit Rates (savings)
  = Cash rate − Bank's margin

When RBA raises rates:
  ├── Variable mortgage rates increase (bad for borrowers)
  └── Savings rates increase (good for savers)

Java Spring Notes — Fee Processing

@Service
public class FeePostingService {

    @Scheduled(cron = "0 0 2 1 * *")  // 2 AM on 1st of each month
    public void processMonthlyAccountFees() {
        List<Account> feeableAccounts = accountRepository
            .findAccountsEligibleForMonthlyFee();
        
        feeableAccounts.forEach(account -> {
            BigDecimal fee = feeCalculator.calculateMonthlyFee(account);
            
            if (fee.compareTo(BigDecimal.ZERO) > 0) {
                ledgerService.postDebit(
                    account.getId(),
                    fee,
                    "Monthly account fee",
                    BankTransactionCode.FEES_MONTHLY
                );
                notificationService.sendFeeAdvice(account, fee);
            }
        });
    }

    @EventListener
    public void onDishonour(DishonourEvent event) {
        BigDecimal dishonourFee = productConfig
            .getDishonourFee(event.getAccountType());
        
        ledgerService.postDebit(
            event.getAccountId(),
            dishonourFee,
            "Dishonour fee - " + event.getPaymentReference(),
            BankTransactionCode.FEES_DISHONOUR
        );
    }
}

account_types.md — Accounts that accrue interest/fees
debit_post.md — How fees are posted as debits
camt054.md — Fee transactions appear in camt.054
camt053.md — Fees appear in end-of-day statement
core_banking.md — Interest engine lives in CBS

Overview​

Interest — The Basics​

Interest Rate Types​

How Interest is Calculated​

Simple Interest (Daily Accrual)​

Compound Interest (Interest on Interest)​

Interest in Banking Systems​

Accrual vs Capitalisation​

Payment Frequency​

Common Bank Fees​

Account Fees​

Payment Fees​

Penalty / Event Fees​

Fee Posting in the System​

Regulation: Responsible Lending & Fee Caps​

Interest on a Home Loan — How It Works​

RBA Cash Rate and Its Effect​

Java Spring Notes — Fee Processing​

Related Concepts​