The Westpac Banking Corp (ASX: WBC) share price is tumbling on Thursday, down 4.12% to $30.48 at the time of writing.

Westpac is significantly underperforming its ASX 200 bank share peers and the broader market today.

The S&P/ASX 200 Index (ASX: XJO) is currently up 0.026%.

Here's how the other major ASX 200 bank shares are performing today:

• National Australia Bank Limited (ASX: NAB) shares are up 0.98% to $36.22
• The Macquarie Group Ltd (ASX: MQG) shares are down 0.5% to $196.73
• Commonwealth Bank of Australia (ASX: CBA) shares are down 0.61% to $164.94
• Australia and New Zealand Banking Group Limited (ASX: ANZ) shares are down 2.89% to $29.12

NAB shares are in the green after the bank released its 1H FY25 results today.

Meanwhile, Westpac shares are floundering for the fourth day in a row.

Here's why.

Why is the Westpac share price down 4% today?

The primary contributor to Westpac's weakness today is that the stock is trading ex-dividend.

It's typical for ASX shares to lose value on ex-dividend day because the entitlement to the next dividend payment is removed.

However, this is the fourth consecutive day that the Westpac share price has fallen, with the catalyst appearing to be its 1H FY25 results.

Westpac released its 1H FY25 results before the market open on Monday.

The Westpac share price finished the day 3.04% lower. On Tuesday, the stock lost another 2.19%. Yesterday, it fell 0.44%. Today, it's down more than 4%.

Let's take a look at the report.

Westpac's half-yearly results

For the six months ended 31 March, Westpac reported net interest income of $9,569 million, up 2% versus the prior corresponding period (pcp).

Westpac's core net interest margin (NIM) was steady at 1.8% on the pcp amid continuing competition in lending and term deposits.

The group NIM declined by one basis point on the pcp to 1.88%.

Non-interest income decreased 3% on the pcp to $1,442 million.

There was 5% pcp growth in loans to $825 billion. This included 5% growth in Australian housing loans, 14% growth in business lending, and 15% growth in institutional lending.

Customer deposits rose by 7% to $697 billion. This included 9% growth in Australian household deposits.

Westpac's operating expenses increased 6% on the pcp to $5,698 million.

The net profit after tax (NPAT) came in at $3,457 million, down 1% on the pcp.

What about dividends?

Westpac announced an interim dividend of 76 cents per share, with 100% franking, which was the same as last year.

The bank will pay the dividend on 27 June.

Westpac investors who would prefer to receive more shares instead of a cash payment have until 5pm AEST on 12 May to submit their dividend reinvestment plan (DRP) elections.

The DRP share price will be the average of the daily volume weighted average market price of Westpac shares sold on the ASX and Cboe Australia between 14 May and 5 June.

What's next for the Westpac share price?

Macquarie has an underperform rating on Westpac and expects its share price to fall by almost 10% from here.

It has a 12-month price target of $27.50 on the stock, which is 9.8% below where Westpac is trading today.

In a new note, Macquarie said Westpac's 1H FY25 result was "only a slight miss to expectations".

However, this provided "a catalyst for the market to review its bullish assumptions on margins, expenses, and dividends".

The broker has an underperform rating on Westpac shares because it thinks the ASX 200 bank stock is expensive.

Macquarie commented:

WBC remains expensive, trading at ~17x FY26E P/E (6-26% premium to ANZ and NAB).

With execution risks around the UNITE program, in addition to headwinds from rate cuts, we continue to see risk to WBC's earnings and multiple.